Code review comment for lp:~robru/phablet-tools/citrain-on-rtm

First of all, thanks for this - it will make our lives a lot easier. Just a couple of queries:

1. If cjwatsons change to software-properties is landing soon, why the hack to write the .list file?
2. The logic to add a PPA was originally in phablet-config writable-image, why are we duplicating it here?
3. Won't adding the PPA without the key break other tools? You should probably fit recv-key for the ci-train-ppa-service in here if you continue with adding the .list file instead of using add-apt-repository