DRAFT: T915: Add MPLS support

[sever-sever]

Add commands op-mode for show mpls.

vyos@mpls:~$ show mpls 
Possible completions:
  ldp           Label Distribution Protocol (LDP)
  pseudowire    Show MPLS pseudowire interfaces
  table         Show MPLS table

vyos@mpls:~$ show mpls ldp 
Possible completions:
  binding       Label Information Base
  discovery     Discovery hello information
  interface     LDP interface information
  neighbor      LDP neighbor information

etc.