Add gnt-node migrate

This is a (modified) forward-port of commit 1190 on the 1.2 branch:

  This is the same as gnt-node failover, and is also a cut&paste of its
  code (almost). It will be really really useful to quickly empty a
  healthy node. I can be persuaded to merge MigrateNode and FailoverNode
  in a common codebase, but could also forget about it and submit it if
  nobody cares.

  Reviewed-by: iustinp

The original MigrateNode function has been converted to the 2.0 style
(cli.JobExecutor). Also commit 2076 has been added that fixes a missing
opcode parameter.

Original-author: ultrotter
Reviewed-by: ultrotter