Add return value to some network API methods

Add return value to api methods add_fixed_ip_to_instance() and
remove_fixed_ip_from_instance(). This allows the @refresh_cache
decorator to use returned nw_info model for updating instance
info cache instead of needing to make another rpc call to get
that same information.

Tests are added to make sure return values are used by the
refresh_cache decorator for methods where the rpc call returns
with the necessary nw_info model.

Change-Id: I748e4db9de62d82e4dbcb0b9c444b16dabf0ec5d
Partial-Bug: #1284345
Merge "Add return value to some network API methods"