Move the policy parsing logic to a separate module.

==
Right now openstack.common.policy module mixes different kind of logic:
* It handle the way json policy file are retrieved (using oslo.cfg for instance)
* It contains the parsing code.
* It defines way rules are checked (the enforcer part).

Besides logging and gettext feature are done using other openstack.common modules.
==

But in some cases user that want to make use of the policy format, may already have ways to retrieve the policy file or already have a logging facility.

So in this case having a separate module that only provide the parsing and checking part would be greatly helpful, and facilitate integration of the policy feature in any other code.

Besides it may facilitate the release of the policy engine as a separate module in the future.

==
Some implementation concerns:
Attention should be taken to not change the external API of the policy.py module.