Verify that instance can boot after volume retype with migration

This tests that an instance can still boot after a volume retype with
migration across backends.

Note: The TestVolumeMigrateRetypeAttached tests only run if there are
multiple backends configured in CINDER_ENABLED_BACKENDS.

Related-Bug: #2019190

Change-Id: Ia88375dd08e0ba2c897fb37c312ef69dab3d001e
1 file changed