Upstart conf: use a pidfile, not the process table

The default upstart config installed with MongoDB doesn't provide the
`--pidfile` or `--make-pidfile` options to `start-stop-daemon`, which
means it determines whether or not MongoDB is running by inspecting the
process table to see if there are any instances of "mongod" running.

This is a pretty crude mechanism, and we can make it substantially more
reliable (not susceptible to mongod instances running inside LXC
containers or similar) by simply getting start-stop-daemon to write a
pidfile.