Adds user_data extension to nova.api.v3.extensions

Although the user_data extension does not create a resource
or controller extension and only implements server.create and
server.create.deserialize entry points it does need to be listed as
a nova.api.v3.extension entry point as the whitelist check
operates on that entry point.

This does not add any runtime overhead and allows user_data to be
specified in the whitelist.

Also adds missing discoverable policy for the user data extension

Partially implements blueprint nova-v3-api

Change-Id: Ia8ac2fa150ab2fe9c1848c5a7ddc416952adf0e4
Merge "Adds user_data extension to nova.api.v3.extensions"