Revert "Merge pull request #416 from axw/mongo-journalenabled"

This reverts commit 62e172632c3e9d8496805ed5223f9f4acc28986a, reversing
changes made to 8275fa461e1644ac073db177c5e456db24991a0c.

The commit being reverted appears to cause mongo to consume large amounts of resources and get stuck when "juju ensure-availability" is run, effectively trashing the environment.