mon/MonmapMonitor: clean up empty created stamp in monmap

mon/MonmapMonitor: clean up empty created stamp in monmap

Some old clusters have an empty created timestamp.  This is mostly
harmless, but it is confusing/wrong, and it does currently break the
telemetry module with errors like

 ValueError: time data '0.000000' does not match format '%Y-%m-%d %H:%M:%S.%f'

from 'ceph telemetry show'.

If we detect an empty created stamp, look at old monmap and use the oldest
modified stamp we can find.

Fixes: http://tracker.ceph.com/issues/39085
Signed-off-by: Sage Weil <sage@redhat.com>