Implement gnt-node evacuate

This patch adds a new 'evacuate' subcommand to gnt-node. The command
will do a replace disks for all instances having that node as secondary
with the new target being the new node given.

The syntax is:
  gnt-node evacuate src_node target_node

The command by itself doesn't do any resource checks, and instead relies
on the LUFailoverInstance code to do that.

Reviewed-by: imsnah