Fix test isolation issues related to versions

Fix test isolation issues related to versions

In cinder.rpc.RPCAPI methods _determine_rpc_version_cap and
_determine_obj_version_cap use global variables LAST_RPC_VERSIONS and
LAST_OBJ_VERSIONS to cache version values so we need to clear these
values between tests so one test values don't get passed to the next
test.

This was problematic for example when running some
cinder.tests.unit.api.contrib tests in a specific order:

- test_admin_actions.AdminActionsTest.test_delete_backup_force_when_creating
- test_backups.BackupsAPITestCase.test_create_backup_snapshot_json

Change-Id: I073557a6e0a17a3e00fd32d48d389a3bcf42e341