Add a cluster parameter to hold the iallocator that will be used by default
when required and no alternative (manually-specified iallocator or
manually-specified node(s)) is given.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>