[upstart] Update upstart configurations (user/start-stop-daemon)

* Introduce 4 parameters in all upstart jobs:
  - COMMAND: Command to run
  - CONF: The configuration file to use
  - USER: Which user to run (user/group)
  - OPTS: Options to pass to COMMAND (default to --config{-,_}file)
* Verify the presence of the configuration file before start
* Allow override of the 4 params with /etc/default/«daemon-name»
* Run Contrail* as contrail (a non privileged user)
* Use start-stop-daemon to launch jobs

Change-Id: I1783540cd8218e8bafb4d0b860ca4c5992f35a19