Use project_reader in security groups tests

Add support for read-only user testing in security groups tests
for SRBAC (Secure RBAC) validation.

Changes:
- Add reader_security_groups_client to BaseSecurityGroupsTest
- Use reader_security_groups_client for all read operations
  (show_security_group, list_security_groups)
- Use reader_servers_client for list_security_groups_by_server
- Keep write operations (create, update, delete) using write client

This follows the existing pattern used for reader_servers_client
and ensures proper SRBAC permission testing.

Change-Id: If365203d918f58611d5a39816564a106500f1b5b
Signed-off-by: Jason Paroly <jparoly@redhat.com>
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
4 files changed