parse: fix 'networkmanager:' backend options for modem connections (#179)
The COMMON_BACKEND_HANDLERS have been forgotten for modem connections apparently. Add them to allow the definition of the special networkmanager: mapping, used for NetworkManager integration. We do not (yet) use that information (like uuid) in the current implementation. But reading YAML via NetworkManager will be broken if the networkmanager: mapping is not accepted.
