Add an optional identity field based on the new DaemonIdentity type
to help name/identify the CephContainers. I would have preferred to make
this field mandatory but there are so many places in the code that
call CephContainer now, I didn't want to have to touch them all at once.
CephContainer objects created using the for_daemon classmethod *will*
all have DaemonIdentity set.
Signed-off-by: John Mulligan <jmulligan@redhat.com>