Add authZ through incorporation of policy checks.

Add authZ through incorporation of policy checks.

Adds the policy openstack-common module and implements policy checks
for the v2 API.  Note that this cut only addresses whole objects (i.e.,
a subnet or a network or a port), not specific fields within objects.
(This means that attributes are not filtered out based on policies.)
Implements blueprint authorization-support-for-quantum.

Change-Id: I1b52b1791a1f14f0af6508a63a40a38e440f15fe