init-ceph: continue after failure doing osd data mount

If we are starting many daemons and hit an error, we normally note it and 
move on.  Do the same when doing the pre-mount step.

Fixes: #8554 Signed-off-by: Sage Weil <sage@inktank.com>
    failed: '$ssh $1'

should be

    failed: '$rootssh $1'