WIP: buildserver: support Docker as a provider

First stab at getting Vagrant working with Docker as the "provider". It creates the base and starts provisioning, but it doesn't finish that yet. The snapshot stuff in fdroidserver/vmtools.py needs to be implemented also, anyone know the right commands to put there? But it seems pretty close.

To use this, do:

$ cd fdroid/fdroidserver
$ echo "vm_provider = 'docker'" >> makebuildserver.config.py
$ ./makebuildserver

closes #119 (closed)

@uniqx @jspricke @proletarius101

(Also, this was largely developed while I was waiting on long running processes in the calyx repo automation, since it means making a small change and waiting a while for the whole thing to run).