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: Ida23a774158f3275bc874d1b52a3424afeeb6fb2