Fix DB access when refreshing the network cache.

When calling ComputeManager.attach_interface(), the following happens:

1) self.network_api.allocate_port_for_instance() is called from within
   ComputeManager.attach_interface();
2) refresh_cache() is called, since it decorates allocate_port_for_instance();
3) refresh_cache calls update_instance_cache_with_nw_info(), with
   "conductor_api=None" since 'conductor_api' cannot be found in the kwargs;
4) update_instance_cache_with_nw_info() calls
   api.db.instance_info_cache_update();
5) Since the call to the database should be made through the conductor, a
   DBNotAllowed exception is raised, which prevents the cache from being updated.

This patch fixes the call to allocate_port_for_instance (step 1) so that
'conductor_api' can be found in the kwargs (step 3).

When using ComputeManager.detach_interface, the exact same problem arises (with
deallocate_port_for_instance instead of allocate_port_for_instance).

This is a first step towards fixing bug #1197192.

Change-Id: I647795907b22790759d3be74afcaf70b712cb338
Merge "Fix DB access when refreshing the network cache."