astakos: Revert bulk update of holdings in commissions

db464c6 introduced a bug: deleting holdings (and then bulk creating them)
interferes with locking; a thread that waits for holdings will not get
the deleted ones.

Revert to separate updating of holdings when issuing and resolving
commissions.
cyclades: Add GANETI_DISKS_WAIT_FOR_SYNC option

Add new setting to denote whether Ganeti will wait for the disk minor to
sync (DRBD). This setting is used when creating a new instance or adding
a new disk to an existing Ganeti instance.