Add rbac_utils is_admin helper method

This patch adds a helper method to rbac_utils.RbacUtils called
``is_admin`` which returns True if the current RBAC test role
is equal to the admin role defined in tempest.conf (i.e.
CONF.rbac.rbac_test_role == CONF.identity.admin_role). This
makes it easier to check this equivalence in a test by simply
running:

    if self.rbac_utils.is_admin:
        do something

Change-Id: I0efb005f90ac77449453f6b68a010ec64e7a03f7
3 files changed