layer_libvirt.py:  noop:  debug output formatting
layer_rbd.py:  move ceph_object tracking to common class

Now tracking works properly.

Also:
- Remove separate snap_exists helper
  - Seems it's not needed anymore?
- noop:  Debug output formatting
layer_libvirt.py:  noop:  debug output formatting
layers.py:  noop:  debug messages