tarmac.conf: keep branch cache in uniform location.

This will make it slightly easier to run the tarmac lander as an
arbitrary user. Only two changes to tarmac-lander.conf and a slight
variation in the instructionsa re needed. These changes (and a new need
to create/chown the branch cache directory) were added to the README
file.
Add a juju charm to deploy certification tarmac.

This simply automates the deployment steps described in the top-level
README.md file.

Note that creating a YAML file with some access keys is required, see
the README file provided. A bit of configuration on Launchpad itself is
also needed.
temporary change to use my configs without disturbing tarmac