diskimage-builder: Test opensuse element for 42.3 in extras-nv
diskimage-builder: Test opensuse element for 42.3 in extras-nv

openSUSE 42.3 is going to be released end of July 2017, so it makes
sense preparing the infra testing for it. Once released we'd like
to switch to 42.3 by default.

Change-Id: I8b30c2b5cd4fee2a0e8dcf792a6547a1656fcabf

openstack-infra/project-config,openstack-infra/project-config