rally verify configure-verifier --extend extra-options.conf converts options to lowercase

`rally verify configure-verifier --extend extra-options.conf` converts options to lower case which case some tests runs with unexpected result. Use `rally verify configure-verifier --show` to
see the extended config file.

Here is an example:

octavia-tempest-plugin has an option RBAC_test_type in load_balancer section:
[load_balancer]
RBAC_test_type = owner_or_admin

the option is defined here:
https://github.com/openstack/octavia-tempest-plugin/blob/a97deefcb3c21194ec1e2b4f11cc0fbd3c16b720/octavia_tempest_plugin/config.py#L99-L103

octavia-tempest-plugin gets this value by using oslo.conf:
https://github.com/openstack/octavia-tempest-plugin/blob/a97deefcb3c21194ec1e2b4f11cc0fbd3c16b720/octavia_tempest_plugin/tests/api/v2/test_availability_zone.py#L99

rally extend extra options by using configparser:
https://github.com/openstack/rally/blob/be72fbb2929c8a08b7a363dd94a6fd7a2c8aef1c/rally/cli/commands/verify.py#L335-L336

configparser converts options to lowercase:
https://docs.python.org/3/library/configparser.html

configparser dose not sensitive to lowercase or uppercase, but oslo.conf does:
$ python
Python 2.7.5 (default, Apr 2 2020, 13:16:51)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-39)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import configparser
>>> conf = configparser.ConfigParser()
>>> conf.read("extra-options.conf")
['extra-options.conf']
>>> conf['load_balancer'].keys()
[u'rbac_test_type']
>>> conf['load_balancer']['rbac_test_type']
u'owner_or_admin'
>>> conf['load_balancer']['RBAC_test_type']
u'owner_or_admin'
>>>

$ python
Python 2.7.5 (default, Apr 2 2020, 13:16:51)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-39)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from tempest import config
>>> CONF = config.CONF
>>> CONF.load_balancer.RBAC_test_type
'advanced'
>>> CONF.load_balancer.rbac_test_type
2020-05-11 14:20:32.483 2619 CRITICAL tempest [-] Unhandled error: NoSuchOptError: no such option rbac_test_type in group [load_balancer]
2020-05-11 14:20:32.483 2619 ERROR tempest Traceback (most recent call last):
2020-05-11 14:20:32.483 2619 ERROR tempest File "<stdin>", line 1, in <module>
2020-05-11 14:20:32.483 2619 ERROR tempest File "/var/lib/kolla/venv/lib/python2.7/site-packages/oslo_config/cfg.py", line 3547, in __getattr__
2020-05-11 14:20:32.483 2619 ERROR tempest return self._conf._get(name, self._group)
2020-05-11 14:20:32.483 2619 ERROR tempest File "/var/lib/kolla/venv/lib/python2.7/site-packages/oslo_config/cfg.py", line 3069, in _get
2020-05-11 14:20:32.483 2619 ERROR tempest value, loc = self._do_get(name, group, namespace)
2020-05-11 14:20:32.483 2619 ERROR tempest File "/var/lib/kolla/venv/lib/python2.7/site-packages/oslo_config/cfg.py", line 3087, in _do_get
2020-05-11 14:20:32.483 2619 ERROR tempest info = self._get_opt_info(name, group)
2020-05-11 14:20:32.483 2619 ERROR tempest File "/var/lib/kolla/venv/lib/python2.7/site-packages/oslo_config/cfg.py", line 3263, in _get_opt_info
2020-05-11 14:20:32.483 2619 ERROR tempest raise NoSuchOptError(opt_name, group)
2020-05-11 14:20:32.483 2619 ERROR tempest NoSuchOptError: no such option rbac_test_type in group [load_balancer]
2020-05-11 14:20:32.483 2619 ERROR tempest
>>>