Assisted Volume snapshot RBAC test for Compute v2.1 API roles

- migrated out common clients and internal methods to the base
- fixes to projects rbac test to use tempest base instead
- migrated volume assistend snapshot rbac tempest test to Patrole

NOTE: This requires a new API client in tempest/lib. A bug is added
for tracking and is used as a reason for skipping.
https://bugs.launchpad.net/tempest/+bug/1668407

Change-Id: I84358aa1e2a76c95a3e5e416770f3053faa2cf3f
1 file changed