Extend/refactor handling of /etc/crowbar/*.json; external SMT support

Extend and refactor handling of selected JSON files in `/etc/crowbar`.  In particular, honour custom provisioner JSON placed in /etc/crowbar/provisioner.json. This will allow us to configure an external SMT server which is referenced by autogenerated autoyast.xml files.  We also need to skip the checks for local repository mirrors on the admin node in order to take advantage of this new functionality.