Move update_instance_info_cache to conductor.

The network API always updates the instance_info_caches table whenever
someone asks for current network information for an instance.  When
the compute manager calls network_api.get_instance_nw_info(), we'll need
to use the conductor for the cache update instead of a local DB call.

Adds an option to network_api's get_instance_nw_info() to not update the
cache and adds code into compute manager to update the cache via the
conductor.

Related to blueprint no-db-compute-manager

Change-Id: I36c3eb5b4e31ef2e26d3023bf2521d95205eb6ac
Merge "Move update_instance_info_cache to conductor."