Update the way of marshalling the transport of network elements when extending the transport with new types

If transport gets extended with new types, this needs to be considered, too. Maybe the suggested way could get implemented how it is.

The following discussion from !530 (merged) should be addressed:

• @hda11249 started a discussion: (+1 comment)

  @istmabauc

  Was thinking about this for a while and am not sure if we should keep storing transportType in the storage or not. Maybe in the future, it would be nice to know if it is gnmi or sth else. But having the option of using a protocol other than gnmi might not happen too soon.