Export unbound variable DIB_RHSM_USER

If 'DIB_RHSM_USER' isn't set, the image build with rhel
element fails because of check 'set -u'. So, we should export
'DIB_RHSM_USER' in script to avoid error during build

Change-Id: Ic63a20e3d91a25bbe804440a5919b035489c60a3
Closes-bug: #1318717
Merge "Export unbound variable DIB_RHSM_USER"