Deprecate instance_get_by_uuid() from conductor

Aside from a few trivial-to-fix places in the unit tests, the
conductor instance_get_by_uuid() method is no longer used. This
deprecates it and cleans up the few places that were trivially
touching it.

Related to blueprint compute-manager-objects-juno

Change-Id: Iab48856c0432af0b9483e9868c2e235de75fdd51
Merge "Deprecate instance_get_by_uuid() from conductor"