Sets policy settings so V3 API extensions are discoverable

Adds the policy settings so the V3 API extensions
are by default discoverable through /v3/extensions. Some of
these policy settings were missed during the porting of
the extensions as the ability to make extensions not discoverable
was done part way through.

Partially implements blueprint nova-v3-api

Change-Id: If2efa15a437d97be8bc026fb04c46ec47eef056f
Merge "Sets policy settings so V3 API extensions are discoverable"