Mechanical change to accommodate explicit jujuMongod for mgo-related tes...

~~This is for part of the solution for https://bugs.launchpad.net/juju-core/+bug/1351019.  It does not fix the issue, but does take a step toward fixing it.~~
This patch accommodates being explicit about where to look for the juju-built mongod binary.  It accompanies https://github.com/juju/testing/pull/27.
I'm torn on this.  While I agree the brackets are unnecessary, isn't it better to avoid code churn later when another var is added?