[Fix gate] Fix failing identity v2 admin tests

This patch fixes the failing identity tests that
belong to the v2 admin API [0].

Due to recent changes to Keystone and DevStack,
the v2 admin API tests fail if run outright.

Tempest now skips the v2 admin tests uness
``CONF.identity_feature_enabled.api_v2_admin`` is true [1];
Patrole should do the same.

This patch makes the following changes:
  - Skips identity v2 admin tests unless
    ``CONF.identity_feature_enabled.api_v2_admin`` is true
  - Removes superfluous tempest.conf overrides from
    post_test_hook
  - Updates ``rbac_utils.switch_role`` to ensure that admin
    identity credentials are properly configured and that
    the ``roles_v3_client`` is always used
  - Refactors test_projects_rbac in v2 identity because
    the API belongs to both the admin and non-admin API,
    which was causing OverPermission error to be thrown.

[0] https://developer.openstack.org/api-ref/identity/v2-admin/
[1] https://review.openstack.org/#/c/458844/

Change-Id: Ic698d0b2cf669793aaad6aff972ba155ef993e4a
11 files changed