Use deactivate-master-ip in cluster-merge

Use the gnt-cluster deactivate-master-ip command in cluster-merge to
disable the master IP.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>