Refactor is_admin
Refactor is_admin

Consolidate duplicate policy checking code. Using the more complete
logic from check_protection also allows assert_admin to work with more
complicated admin_required rules.

Change-Id: I49c7c9d2732675b87c58c9460671dfe13969fd34

openstack/keystone,mahak/keystone,mahak/keystone,mahak/keystone,rajalokan/keystone,rajalokan/keystone,openstack/keystone,openstack/keystone,rajalokan/keystone