Use the right mongod binary on utopic

Use the right mongod binary on utopic

There was some broken code in agent/mongo
for calculating the mongod path. It special
cased trusty in two different ways.

Fixes lp:1321025

https://code.launchpad.net/~axwalk/juju-core/lp1321025-mongod-path-1.18/+merge/220544

(do not edit description out of merge proposal)

[axw, 2014-05-22 01:59:43]

Please take a look.

[wallyworld, 2014-05-22 02:03:16]

LGTM