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
diff --git a/jenkins-config/clouds/openstack/cattle/ubuntu1804-builder-2c-2g.cfg b/jenkins-config/clouds/openstack/cattle/ubuntu1804-builder-2c-2g.cfg
index 6195074..1cf4672 100644
--- a/jenkins-config/clouds/openstack/cattle/ubuntu1804-builder-2c-2g.cfg
+++ b/jenkins-config/clouds/openstack/cattle/ubuntu1804-builder-2c-2g.cfg
@@ -1,3 +1,4 @@
 IMAGE_NAME=ZZCI - Ubuntu 18.04 - builder - x86_64 - 20200317-165726.675
 LABELS=ubuntu1804-builder-2c-2g
 HARDWARE_ID=v2-highcpu-2
+VOLUME_SIZE=20