Don't deallocate/reallocate networks on reschedules

If network allocation is not host dependent we don't need to deallocate
and reallocate a network for each reschedule.  Only baremetal has host
dependent allocation so for that driver networks are de/reallocated on a
reschedule.

The current method in the code which checks for the length of the list
returned from _get_instance_nw_info() doesn't work because it can't
distinguish between an unallocated network and one that doesn't return
network information.

Part of bp remove-cast-to-schedule-run-instance

Change-Id: Ic427715f46cb5179a37c593c4217f239c65544c3
Merge "Don't deallocate/reallocate networks on reschedules"