Make compute/manager use conductor for unrescue()

The compute_api.unrescue() call makes direct database queries, which means
it can't be called directly from compute manager. This patch adds support
in conductor for proxying this request and makes compute manager utilize
it.

Fixes bug 1167619

Change-Id: I158c64cabdfe436462f5669047fb930819a13c50
Merge "Make compute/manager use conductor for unrescue()"