If a node (or nodetype) has the delayreloadtillalloc attribute set, then
in nfree the partitions are cleared and the node is powered off. Reload
is being delayed till the next time the node is allocated to an
experiment.

Add -Z option to os_load, which is like -P, but applies the clear
metadata option to all disks.

os_setup and os_load have been changed accordingly.