Make RHSM registration set -u safe
Make RHSM registration set -u safe

There are a bunch of optional env vars references in the rhsm
registration script, but they weren't wrapped to be set -u safe so
they cause build failures if any aren't set.

Change-Id: I18326b618f0920efa4dbd805611c6ee66d33ebb0

CSC-IT-Center-for-Science/diskimage-builder,eantyshev/diskimage-builder,zioproto/diskimage-builder,varunarya10/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,eantyshev/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,switch-ch/diskimage-builder,switch-ch/diskimage-builder,varunarya10/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,JioCloud/diskimage-builder,NaohiroTamura/diskimage-builder,NaohiroTamura/diskimage-builder,rdo-management/diskimage-builder,rdo-management/diskimage-builder,JioCloud/diskimage-builder,takahashinobuyuki/diskimage-builder