switch to the newly created 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.

This is the remaining work to be done after https://review.opendev.org/#/c/667539
got merged earlier

Change-Id: I3c0e14c606c88b5deecfad71bc156e53335d543e
1 file changed