Replace openSUSE experimental check with newer version

We're migrating now to openSUSE 15.0 (yes, 15.0 is newer than 42.3),
remove legacy job and replace with native Zuul v3 job running python3.

Change-Id: Ic7af06ed7dfe4bd95c6f9cf0a24bfd986b6ad8f5
Depends-On: https://review.openstack.org/609359

.zuul.yaml

@@ -274,7 +274,7 @@
               - ^releasenotes/.*$
               - ^setup.cfg$
               - ^tox.ini$
-        - legacy-tempest-dsvm-neutron-full-opensuse-423:
+        - tempest-full-py3-opensuse150:
             irrelevant-files:
               - ^(test-|)requirements.txt$
               - ^.*\.rst$