Tempest config doesn't allow None values

The below command works in python2.7, however in python3.7 this error is introduced.
This has been traced down to an issue with how the python3.7 builtin configparser works (confirmed with @andreykurilin on gitter).

https://docs.python.org/3.7/library/configparser.html it states "allow_no_value, default value: False", so I'm passing that value as true would resolve this issue.

https://github.com/openstack/rally-openstack/blob/master/rally_openstack/verification/tempest/config.py#L46

self.conf = configparser.ConfigParser()

should be changed to

self.conf = configparser.ConfigParser(allow_no_value=True)

Command:
rally verify start --id tempest_verifier --pattern set=identity --detailed --deployment-id os_deployment

Results from command:
error: 2020-02-18 23:29:45.243 9724 INFO rally.api [-] Configuring verifier 'tempest_verifier' (UUID=07f3fd49-6771-4bfc-a8ec-f8707f75cfd3) for deployment 'os_deployment' (UUID=1eb3d732-fb10-47ab-81c7-a28a1d487e4b). option values must be strings

Results from command with -debug:
09:42:46.702 14432 RALLYDEBUG rally.api [-] INFO logs from urllib3 and requests module are hide.
2020-02-19 09:42:46.703 14432 RALLYDEBUG rally.api [-] urllib3 insecure warnings are hidden.
2020-02-19 09:42:46.703 14432 RALLYDEBUG rally.api [-] ERROR log from boto module is hide.
2020-02-19 09:42:48.071 14432 INFO rally.api [-] Configuring verifier 'tempest_verifier' (UUID=3d619624-45cb-4f75-a148-4980da42eef0) for deployment 'os_deployment' (UUID=6a391481-e088-42c9-9457-3d75940909fe).
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils [-] Unexpected exception in CLI: TypeError: option values must be strings
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils Traceback (most recent call last):
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally/cli/cliutils.py", line 674, in run
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils ret = fn(*fn_args, **fn_kwargs)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "</usr/local/lib/python3.7/dist-packages/decorator.py:decorator-gen-93>", line 2, in start
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally/cli/envutils.py", line 68, in default_from_global
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils return f(*args, **kwargs)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "</usr/local/lib/python3.7/dist-packages/decorator.py:decorator-gen-92>", line 2, in start
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally/cli/envutils.py", line 68, in default_from_global
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils return f(*args, **kwargs)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "</usr/local/lib/python3.7/dist-packages/decorator.py:decorator-gen-91>", line 2, in start
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally/plugins/__init__.py", line 51, in ensure_plugins_are_loaded
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils return f(*args, **kwargs)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally/cli/commands/verify.py", line 519, in start
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils tags=tags, **run_args)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally/api.py", line 1016, in start
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils deployment_id=deployment_id)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally/api.py", line 860, in configure
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils raw_config = vm.configure(extra_options=extra_options)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally_openstack/verification/tempest/manager.py", line 107, in configure
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils return tcm.create(self.configfile, extra_options)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally_openstack/verification/tempest/config.py", line 211, in create
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils method()
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/local/lib/python3.7/dist-packages/rally_openstack/verification/tempest/config.py", line 75, in _configure_identity 2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils self.credential.region_name)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/lib/python3.7/configparser.py", line 1197, in set
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils self._validate_value_types(option=option, value=value)
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils File "/usr/lib/python3.7/configparser.py", line 1182, in _validate_value_types

Daniel Lutton @dlutton 01:50
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils raise TypeError("option values must be strings")
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils TypeError: option values must be strings
2020-02-19 09:42:48.179 14432 ERROR rally.cli.cliutils

The error indicates an issue with self.credential.region_name. When I include this value as a credential, the rally command works. With python2.7 this parameter could be left out, however now it's required.