Don't run test_volume.VolumeTestCase twice

Don't run test_volume.VolumeTestCase twice

Currently the test_volume.VolumeMigrationTestCase class inherits
from test_volume.VolumeTestCase and therefore runs all the unit tests
in the former class in addition to the 29 tests defined in its
own class.  This provides no gain in coverage and is costly since
the tests in VolumeTestCase are among the most expensive we have.

This commit refactors cinder/tests/unit/test_volume.py such that
VolumeMigrationTestCase inherits directly from BaseVolumeTestCase,
thereby reducing the number of tests run from test_volume from
557 to 412, and total execution time (on my system) from 335s to
202s.  Only duplicate tests are removed.

Depends-On: Ia70e51719abb9a6ed357802446847ffa81d7427e

Change-Id: Ief4706f50838e1f906119992302a39a6014126b3