[openstack|compute] update flavor tests

- Update mocks and reorder tests so both live and mocked tests pass.
- Ensure #remove_flavor_access sends `tenant_id` as a String.
  If sent as an Integer, the server will return 500.
  (#add_flavor_access works either way)
Merge pull request #2070 from burns/openstack_compute_flavor_tests

[openstack|compute] update flavor tests