This changes the way oslo.policy and oslo.context are used. This aims at
delegating as much work as possible to these libraries in order to ease
maintainability.
Work items:
* Use oslo_context.RequestContext's from_environ() method instead of building
the context manually.
* Replace "tenant_id" with "project_id" in policy rule definition/enforcement.
* Introduce a cloudkitty.context module allowing to easily tweak request
context creation.
Change-Id: I91f38b9aded2dc6453507edaa55cfd9ddf7e995b
a80d0e977d