Add use_external_mip_script cluster parameter

Add the use_external_mip_script cluster parameter, that represents
whether the master IP address turnup/turndown procedures must use a
script provided by the user (True) or the one provided by Ganeti
(False).

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>