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: I87ad1e083ce0b826a40282c0e4b6cebc3d32af30
Depends-On: https://review.openstack.org/609359

.zuul.yaml

@@ -216,7 +216,7 @@
               - ^keystone/tests/unit/.*$
               - ^releasenotes/.*$
               - ^setup.cfg$
-        - legacy-tempest-dsvm-neutron-full-opensuse-423:
+        - tempest-full-py3-opensuse150:
             irrelevant-files:
               - ^(test-|)requirements.txt$
               - ^.*\.rst$