Consolidate RBAC test setup in single base class

It would be better to create a single base class from which all other RBAC base classes (i.e. the one for compute, network modules) inherit. This base class will do commonly used RBAC setup (i.e. initializing rbac_utils). Explore migrating logic related to dynamic file discovery and service validation out of rbac_utils into this base class, using the appropriate Tempest pattern for initializing clients, etc.