Refactor policy parser init so that validate service is in helper

Refactor the rbac_policy_parser __init__ function so that service
validation is done in its own helper to make the code more
readable.

This commit specifically:
  - refactors the code to make it more readable by creating the
    validate_service helper in ``RbacPolicyParser``
  - makes validate_service a classmethod so that the list of
    services can be cached in memory to avoid list_services
    call for every single test
  - this is a light memory footprint because there will only
    ever be a finite (sub-100) number of services running

Change-Id: I6f6a57d5014e0e2aa373c1b099a6bcc371bec165
2 files changed