Add scope_types to policy association policies
Add scope_types to policy association policies

This commit associates `system` to all policy association policies.
Since policy association can be done to endpoints and services, which
are system-level resources, it makes sense to require system-scoped
tokens to make those associations.

From now on, a warning will be logged when a project-scoped token is
used to make policy associations to endpoints or services. Operators
will have the option to require system-scoped tokens for these
policies by enabling oslo.policy's `enforce_scope` configuration
option.

Change-Id: I18692cd14f9c57816da173038d7dd9ff0802246a

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