Don't update API cell on get_nwinfo

Every call to get_instance_nw_info() updates an instance's info_cache.
The purpose of this is to make sure we're constantly healing the
info_cache table.  However, this causes a lot of unneeded updates
traveling up to the API cell.  Cells has its own periodic task that
heals all instance data.

Fixes bug 1180304

Change-Id: I07774712a6358999fd1fdd0f5a90c6f05e3d8d6e
Merge "Don't update API cell on get_nwinfo"