Prevent readding of the master node

This breaks Ganeti in multiple ways. If we don't make the check in
gnt-node itself, then bootstrap.SetupNodeDaemon will restart the
master daemon, making the operation fail:

  node1# gnt-node add --readd node1
  Cannot communicate with the master daemon.
  Is it running and listening for connections?

The check in cmdlib is more of a safety check, as we shouldn't reach
it. If we do (via a bad client), then it will prevent breakage in the
job queue/config handling.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>