cthulhu: Fix intermittent exception on new cluster
cthulhu: Fix intermittent exception on new cluster

There was a brief window between construction and the
first call to on_heartbeat where the update_time
attribute of ClusterMonitor was None, causing an
exception rendering to a datetime for the API.

Signed-off-by: John Spray <a0ab4ec42db5a70bbcbfc74d8971c0305528d92b@inktank.com>

r0h4n/ceph-integration