ceph_common.sh: add warning if 'host' contains dots

This is a common error and there's no reason the script can't
at least tell you it's a really bad idea.  One might argue it
could even successfully proactively truncate the host parameter
at the first dot, but that's a little controlling, perhaps.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>