Refactors Patrole framework to only use admin tenant credential type.

This patch adds following capablities to RBAC tempest framework:

1. Restricts admin only tenant to be used for rbac as much as possible,
   and thus removes need of creating additional tenant with "primary"
   credential type.
2. Patrole framework requires "tempest_roles" value in the conf file
   to be set to "admin" role. Which again restricts tempest.conf to a
   hardcoded value. This patch takes care of this problem also.

Note: Adding this patch will required some cleanup and refactoring in
test files, which will be taken care in separate commits component wise.

Co-Authored-By: Mh Raies <mh.raies@ericsson.com>
Co-Authored-By: Felipe Monteiro <felipe.monteiro@att.com>

Implements bp: modifying-switching-role-mechanism
Closes-Bug: #1664600
Closes-Bug: #1664278

Change-Id: Ic665d35332def6b6ec7b0065d1ebe65514a926b9
80 files changed