ssh: use provider => 'upstart'

Apparently, on trusty, the behavior of the init.d SSH script has
changed, and instead of shelling out to /lib/init/upstart-job when
upstart is detected, it exits 1 instead on both "status" & "start"
actions. Use provider => upstart instead, to make puppet use upstart
directly.

Change-Id: Ifb657c76c1b0d127acb262eee887d20d5919ba31