Implement system member test coverage for groups
Implement system member test coverage for groups

This commit introduces explicity test coverage for system members,
making sure they are allowed to perform readable and not writable
group operations.

Subsequent patches will incorporate:

  - system admin functionality
  - domain reader functionality
  - domain member test coverage
  - domain admin functionality

Change-Id: Ie22a18ac7b243089509001fda930474f55e29d3f
Related-Bug: 1805369
Related-Bug: 1808859
Related-Bug: 968696

openstack/keystone,mahak/keystone,mahak/keystone,openstack/keystone,mahak/keystone,openstack/keystone