Set DIB_COMMON_ELEMENTS for devtest_overcloud.sh
Set DIB_COMMON_ELEMENTS for devtest_overcloud.sh

DIB_COMMON_ELEMENTS is now expected by devtest_overcloud.sh for common
elements that should be in all images. Setting so that images include
pypi-openstack, pip-cache and stackuser.

Change-Id: I219875b1b0e943f0cdf45684a7ca06244c6107fa

rdo-management/tripleo-image-elements,openstack/tripleo-image-elements,radez/tripleo-image-elements,radez/tripleo-image-elements,openstack/tripleo-image-elements,rdo-management/tripleo-image-elements