With the recent changes, we were leaving the
installation of mongodb until too late. We now
ensure that mongodb is installed early and we
do this unconditionally.

(Cherry pick from master)

Fixes https://bugs.launchpad.net/juju-core/+bug/1350700
Merge pull request #446 from axw/lp1350700-mongo-install-mongodb-always-1.20

mongo: always install mongodb in EnsureServer

With the recent changes, we were leaving the installation of mongodb until too late. We now ensure that mongodb is installed early and we do this unconditionally.

(Cherry pick from master)

Fixes https://bugs.launchpad.net/juju-core/+bug/1350700