Adds v3 API extension discovery filtering

Adds ability for a v3 loaded extension to be visible in
/v3/extensions to be dependent on the discoverable action policy
for that extension. Note that this does not actually effect whether
or not the functionality provided by the extension is accessible

Implements blueprint nova-v3-api-filter

Change-Id: I3f6ba7ea59e7abfa9d57b79ab18aa5b675e64118
Merge "Adds v3 API extension discovery filtering"