upstart: simplify start; allow group stop via an abstract job

upstart: simplify start; allow group stop via an abstract job

Use a 'ceph-mds' or 'ceph-mon' event to start instances instead of
explicitly calling start.  This avoids the ugly is-this-already-running
check.  [Thanks Guilhem Lettron for that!]

Make the -all job abstract (which means it stays started and can be
stopped).  Trigger a helper task (-all-starter) to trigger instance
start.  Make instances stop with the -all task.  This allows you to do

 start ceph-mds-all
 stop ceph-mds-all
 start ceph-mds id=foo
 start ceph-mds-all
 stop ceph-mds id=bar
 stop ceph-mds-all

but not

 start ceph-mds id=foo
 stop ceph-mds-all

because ceph-mds-all isn't running.  Not quite as flexible in sysvinit in
that regard, but good enough for me.

Fixes: #2414
Signed-off-by: Sage Weil <sage@inktank.com>