Add cgmanager readiness notification for upstart.

Technically requires Upstart 1.13 which supports the 'cgroup' stanza
but make it a soft reference by using '|| true' to avoid failing to
start the cgmanager/cgproxy jobs on upstart systems that are either
older than 1.13 or which have been built without cgroup support.

See:

- https://code.launchpad.net/~jamesodhunt/ubuntu/utopic/cgmanager/enable-upstart-cgroup-support/+merge/227209
- https://launchpad.net/upstart/+announcement/12821
- http://upstart.ubuntu.com/cookbook/#initctl-notify-cgroup-manager-address