Add 'use_slave' to instance_get_all_by_filter in conductor

When the use_slave parameter was added to instance_get_all_by_filter in
the db layer and nova object layer it was missed in conductor, causing a
failure in the _poll_rescued_instances periodic task.  This adds the
parameter where necessary.

Change-Id: I16ab29730d72b2e2941766b5b12568976facec39
Closes-bug: 1279358
Merge "Add 'use_slave' to instance_get_all_by_filter in conductor"