Make compute manager use network api method return values

Calls to network api methods add_fixed_ip_to_instance() and
remove_fixed_ip_from_instance() return updated nw_info models
so there is no need to make separate calls to get this info.

This change refactors the affected code to avoid such extra
calls. Related methods in the neutron api are updated to return
the same information as their nova-network api counterparts.

Change-Id: I39dcf257ed5c101b13c2243266eed265c64e3c98
Closes-Bug: #1284345