upstart: only start when 'upstart' file exists in daemon dir

upstart: only start when 'upstart' file exists in daemon dir

We need to distinguish between daemons managed by upstart and sysvinit
(and, eventually, systemd).  Only start daemons when 'upstart' is present.

Note that sysvinit will only start daemons when the 'host = ...' line is
in ceph.conf, so there is a similar "opt-in".

Signed-off-by: Sage Weil <sage@inktank.com>