Make resource tracker use conductor for listing compute nodes

This makes the resource tracker use the existing conductor method
of service_get_all_compute_by_host() instead of querying the
database directly.

Related to blueprint no-db-compute

Change-Id: Iee19eeaa8651e4df0abb974f3f3ddeafd9d23409
Merge "Make resource tracker use conductor for listing compute nodes"