Switch to opensuse-15 nodeset

opensuse-150 nodeset is referring to openSUSE 15.0, which is still in
maintenance but openSUSE 15.1 has been released already. "opensuse-15"
is going to refer to the "latest openSUSE 15.x" build released and
working for OpenStack going forward, so add this nodeset and use
it by default going forward.

The new job tempest-full-py3-opensuse15 use the opensuse-15 nodeset,
change tempest-full-py3-opensuse150 to tempest-full-py3-opensuse15.

Change-Id: Ia76b107a137a39716253631e5a8d294f7ee9614e

.zuul.yaml

@@ -284,7 +284,7 @@
             irrelevant-files: *tempest-irrelevant-files
         - tempest-pg-full:
             irrelevant-files: *tempest-irrelevant-files
-        - tempest-full-py3-opensuse150:
+        - tempest-full-py3-opensuse15:
             irrelevant-files: *tempest-irrelevant-files
     periodic:
       jobs: