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.

Fixes https://bugs.launchpad.net/juju-core/+bug/1350700
Merge pull request #445 from axw/lp1350700-mongo-install-mongodb-always

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.

Fixes https://bugs.launchpad.net/juju-core/+bug/1350700