MongoDB: don't fork; let upstart drive.

Currently 'initctl status mongodb' always reports a status of 'stop/waiting'
for MongoDB. Consequently, every time Puppet runs, it thinks MongoDB has
stopped and it restarts it. This is because MongoDB has been configured to
daemonize, but the upstart configuration does not include an "expect daemon"
statement, so it loses track of the process. The solution is either to add
"expect daemon" to the upstart config or to tell MongoDB not to fork and
instead allow upstart to daemonize it. I went with the latter approach because
it was easy to do using our current MongoDB module structure.

Change-Id: I73cf30d9a2e8eb04ce38013539c0a23b90e5459d