Update tests for scoped tokens and default roles

This patch refactors the RBAC enforcement checks in the API tests.
It also updates those test for keystone scoped tokens and default roles.

Change-Id: I6fad03f5a89c213562918ca258884aac34ba7ce7
diff --git a/octavia_tempest_plugin/common/constants.py b/octavia_tempest_plugin/common/constants.py
index 927aa84..aea476e 100644
--- a/octavia_tempest_plugin/common/constants.py
+++ b/octavia_tempest_plugin/common/constants.py
@@ -183,6 +183,7 @@
 
 # RBAC options
 ADVANCED = 'advanced'
+KEYSTONE_DEFAULT_ROLES = 'keystone_default_roles'
 OWNERADMIN = 'owner_or_admin'
 NONE = 'none'