Set Ubuntu 18.04 builders to 20GB volume, take 2

The new Ubuntu images that we recently changed over to require at
least 20GB of HDD, rather than the previous setting of 10GB.
Previous attempt removed the VOLUME_SIZE lines entirely with the
intent of using a default value, but that did not work post merge.

Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>
Change-Id: Ica06b982e7e6d2ce61e802be13d2e92638635b12