labs: add BASE_DISK_SIZE configuration option

A new configuration option, BASE_DISK_SIZE, allows users to change the
size of the disk available to the node VMs.

Because the disks grow dynamically, the cost of larger disks is limited
in practice to additional time spent in zero_empty.sh.

Change-Id: I47a72a08cfa51ba6c04306b64e04c4c2600975dd