Now that oslo.context has been bumped to >=2.6.0,
we can use `is_admin_project` from the context which
is backward compatible.
This also adds a new rule `project_admin` to make
resource types accessible inline with current policy
of other services like nova, that are yet to use the
`is_admin_project` feature. Once those services start
using the is_admin_project feature, we can remove this.
Change-Id: I5be8176042f8839e86f77984222e7fac66dfaed6
Related-Bug: #1466694
168a7797e5