Fix galera_server installation for openSUSE Leap 15.1
Fix galera_server installation for openSUSE Leap 15.1

The MariaDB repositories are offering a download repo under
the leap major version, e.g. "opensuse15" or "opensuse42" for all leap
42.x and leap 15.x versions, respectively. There isn't a specific
build for openSUSE Leap 15.1, so the previous logic selected
a repo url of opensuse-151 which does not exist.

Change-Id: Ia0985c03d51c3bfe06b98ae73a3533c3402453e3

openstack/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server