fix for 1.18-1.19 upgrade failure

[Nate Finch]

If anyone has some time to do a very quick review, this change lets us
upgrade from 1.18 to 1.19.

The problem was that trunk was replacing the mongo upstart script of
existing environments to start mongo with --replSet, but we can't actually
support that just yet.  So the fix is just to not replace the old upstart
scripts until we do support that kind of upgrade.

https://codereview.appspot.com/86930046

-Nate
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.ubuntu.com/archives/juju-dev/attachments/20140411/6efa332a/attachment.html>