Nimble unit tests set invalid trace flags

Running a unit test such as cinder.tests.unit.volume.drivers.test_nimble.NimbleDriverVolumeTestCase.test_delete_volume

will produce errors:

2017-05-17 14:36:28,287 WARNING [cinder.utils] Invalid trace flag: M
2017-05-17 14:36:28,287 WARNING [cinder.utils] Invalid trace flag: L
2017-05-17 14:36:28,288 WARNING [cinder.utils] Invalid trace flag: N
2017-05-17 14:36:28,288 WARNING [cinder.utils] Invalid trace flag: I
2017-05-17 14:36:28,288 WARNING [cinder.utils] Invalid trace flag: E
2017-05-17 14:36:28,288 WARNING [cinder.utils] Invalid trace flag: B

This is because of how configuration is mocked out: def create_configuration() sets safe_get.return_value to 'NIMBLE', meaning this value is returned for many config options it is not valid for.

This config mocking should be tightened up and only use valid values for config options.