Fix cluster-merging by not stopping noded

cli.RunWhileClusterStopped() stops noded on all of the nodes in the
original cluster. This prevents /etc/hosts updates on the master, and
config redistribution doesn't reach the other nodes in the original
cluster. As all we want to do is merge while the master is stopped,
simply stop it and start it again after.

Signed-off-by: Stephen Shirley <diamond@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>