Add and 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.

Change-Id: Ic3f4d6998a66da5226bc95088d7e3c83dfe737ce
diff --git a/stack.sh b/stack.sh
index 9bda79a..20938ad 100755
--- a/stack.sh
+++ b/stack.sh
@@ -224,7 +224,7 @@
 
 # Warn users who aren't on an explicitly supported distro, but allow them to
 # override check and attempt installation with ``FORCE=yes ./stack``
-if [[ ! ${DISTRO} =~ (xenial|artful|bionic|stretch|jessie|f28|f29|opensuse-42.3|opensuse-15.0|opensuse-tumbleweed|rhel7) ]]; then
+if [[ ! ${DISTRO} =~ (xenial|artful|bionic|stretch|jessie|f28|f29|opensuse-15.0|opensuse-15.1|opensuse-tumbleweed|rhel7) ]]; then
     echo "WARNING: this script has not been tested on $DISTRO"
     if [[ "$FORCE" != "yes" ]]; then
         die $LINENO "If you wish to run this script anyway run with FORCE=yes"