upstart: pass '--pid-file' param to udp2log

The upstart config currently generates a value for $PIDFILE and passes it to
start-stop-daemon, which then expects the process to create the pidfile.
udp2log does not create the pidfile because it is not told to. As a result,
upstart is unable to correctly infer the state of the udp2log daemon. (Reported
by Ottomata.)

Untested.

Change-Id: I3589638eec21f5075c3f18328dc55a2649d2477a