Implement system member role domain test coverage
Implement system member role domain test coverage

This commit introduces the system member role to the API, making sure
system members can execute readable operations, leaving writable
domain operations to system administrators.

Subsequent patches will include domain support for:

  - system admin functionality
  - domain user test coverage
  - project user test coverage

Change-Id: I1d21ba562b007b43fc36a7a2010d35591ca3bae5
Partial-Bug: 1794376
Partial-Bug: 968696

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