Add juju mongod

This is part of the fix for https://bugs.launchpad.net/juju-core/+bug/1351019.  It eliminates a hard-coded path.
Consistency of style across a large code base / dev team is important. This comment does not comply with the style guide (docs/contributions/style-guide.md):

```
Comments should be full sentences with proper capitalization and punctuation. 

Avoid adding non-useful information, such as flagging the layout of a file with 
block comments or explaining behaviour that is immediately evident from reading 
the code.
```

 If you believe we should be using comments as visual separators (which I do not believe is a current Go or Juju convention), please make a pull request, with a justification, to the style guide.