vtysh: add the "show yang operational-data" command

vtysh: add the "show yang operational-data" command

The right way to implement this command in vtysh is the following:
* Send the command to each running FRR daemon;
* Collect the command output from each daemon;
* Parse the text outputs into libyang lyd_node structures. Then merge
  all these data trees into a single one. Finally, print the merged
  data trees to the standard output (libyang will take care of
  combining duplicate nodes as necessary).

What this commit does is to allow vtysh to send the "show yang
operational-data" command to a single daemon only (the last
parameter).  It's a quick workaround to allow us to write topotests
using YANG-modeled data until we do the real thing (full vtysh
northbound integration).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>