Add initial code to support policy.json implementation

We don't currently support a policy.json file like
other openstack services, so this code (mostly copied
from glance, then modified a bit) will allow us to
add policy-based authorization to out APIs fairly
easily

Change-Id: I5ad9f55b3d0979e2526953bdce8b8227852e4b72
Signed-off-by: Steven Hardy <shardy@redhat.com>