Add extension access_ips for v3 API

The functionality of access ips overlaps heavily with floating ips,
and the most deployments don't use access ips now. So we decided to
demote access_ips to extensions, the more detail discussion at:
https://etherpad.openstack.org/NovaV3APICore

New extension add prefix 'os-access-ips' for properties 'access_ip_v4' and
'access_ip_v6' to follow v3 API rules. And add unittest for new extension.

For making this patch small, this patch just add the code of extension
and related unittest, without enable the new extension and remove the
old access_ips attributes. That will be done by next patch.

If you want to test this new extension, you can enable it by adding
'access_ips = nova.api.openstack.compute.plugins.v3.access_ips:AccessIPs'
to entry point 'nova.api.v3.extensions' and
'nova.api.v3.extensions.server.create*/rebuild*/update' in setup.cfg.

DocImpact

Partially implements bp nova-v3-api

Change-Id: Ib56e4ff9145798792f062fc107390565673ac9a1
Merge "Add extension access_ips for v3 API"