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: If7e1b1108acb7a7f8b6b7524213ba5dae5560b53
Depends-On: https://review.openstack.org/609359

.zuul.yaml

@@ -321,7 +321,7 @@
               - ^setup.cfg$
               - ^tools/.*$
               - ^tox.ini$
-        - legacy-tempest-dsvm-neutron-full-opensuse-423:
+        - tempest-full-py3-opensuse150:
             irrelevant-files:
               - ^(test-|)requirements.txt$
               - ^.*\.rst$