extract json_edit variable for brevity
extract parse_node_data variable for brevity
rename CROWBAR_FILE to CROWBAR_JSON for consistency
add json_read
avoid duplication when handling custom JSON, and honour custom provisioner JSON

This will allow us to configure an external SMT server by placing appropriate
configuration in /etc/crowbar/provisioner.json, e.g.:

{
  "attributes": {
    "provisioner": {
      "inst_base_urls": {
        "sles":       "http://smt.mydomain.com/repos",
        "suse_cloud": "http://smt.mydomain.com/repos"
      }
    }
  }
}
skip local repo checks if external SMT server is configured