Use conductor for instance_info_cache_update.

This patch updates the network API to be able to use the conductor API
to be able to update the instance info cache.

A previous commit, 5731852f91c32fe09db541888d3fee9e3139fc43, updated
the compute manager to use this method from conductor when getting
network info.  This patch builds on that by moving the conductor usage
down to the network API and using it in more places.  Specifically, the
network API calls used by the compute manager that use the refresh_cache
decorator were updated.

The refresh_cache decorator now checks for a conductor_api kwarg and
will use the conductor to update the cache if it's not there.  Otherwise
it goes back to default behavior of using the db directly.

Change-Id: Ib5e17f5f8fb4ee14e3ed305057d18d40f2e905e5
Merge "Use conductor for instance_info_cache_update."