Make resource tracker use conductor for listing instances

This patch extends the conductor manager's get_all_by_host() method
to take an optional node parameter, which will cause the additional
filtering, if present. It also makes resource tracker use the
conductor for getting instances on the current host/node.

Related to blueprint no-db-compute

Change-Id: I407757d356b4162f32da29924175175a40f88f13
Merge "Make resource tracker use conductor for listing instances"