policy.pp : Fix duplicate ressource issue openstacklib::policy

Currently, in the policy.pp file the class openstacklib::policy
is called. The same apply for all components resulting in a
Duplicate Ressource issue. Using directly the underlying ressource
openstacklib::policy::base we prevent this issue.

Change-Id: I7c71e957fbae4caa7a4fc3309c58a0abd057e412