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
2 files changed