Hard code logdir/datadir to Ubuntu paths

We should never use the client's OS to
decide how to form paths for the target.
This change hardcodes an Ubuntu series
to generate the paths for now, until
Windows support is in.

Fixes https://bugs.launchpad.net/juju-core/+bug/1342725
Hmm, i wonder if people running on precise would find this confusing. Should we use the series if on Ubuntu? I think there's a helper function somewhere for this sort of thing?