Change LOG.warn to LOG.debug in _shutdown_instance

Change LOG.warn to LOG.debug for detaching volume related exceptions
in _shutdown_instance in compute/manager.py, since as the log message
said the exceptions should be ignored. It's probably not worth a
warning in the log.

Change-Id: Ibe216b9bccf590e64aa1b966753bd1ec67c8dbdf
Merge "Change LOG.warn to LOG.debug in _shutdown_instance"