test_stamp_pattern times out waiting for the device to show up on the guest

When running test_stamp_pattern sometimes the test times out waiting for the device to show up on the guest after the volume attach is issue. An example traceback is:

     Traceback (most recent call last):
       File "tempest/test.py", line 103, in wrapper
         return f(self, *func_args, **func_kwargs)
       File "tempest/scenario/test_stamp_pattern.py", line 139, in test_stamp_pattern
         keypair['private_key'])
       File "tempest/scenario/test_stamp_pattern.py", line 89, in _wait_for_volume_available_on_the_system
         raise lib_exc.TimeoutException
     tempest.lib.exceptions.TimeoutException: Request timed out
     Details: None

This failure was mode is why we originally so hesitant to unskip the test, and it doesn't look like it's better. The logstash query:

http://logstash.openstack.org/#dashboard/file/logstash.json?query=message%3A%5C%22_wait_for_volume_available_on_the_system%5C%22

shows 49 hits on master and stable/mitaka since the test has been unskipped