Create CLI to set direct or vxlan mode

[acloudiator]

Current procedure is to set the directmode with etcdctl (etcd client), it's working well. But for ease of management, we might look into possibilities to set this with polycubed cli.

[acloudiator]

I talked to @frisso, we will implement it, however, we don't have a planned date for it yet.
-Mauricio