Drop openSUSE from commonly tested distro list

OpenSUSE is no longer maintained for testing
in OpenStack upstream CI, devstack job is broken
for more than month and unfortunately we do not have
any active maintainer to fix and maintain it.

Devstack team is planning to drop the openSUSE job
- https://review.opendev.org/c/openstack/devstack/+/769884

If we cannot test it with working job then we should remove
it from wallaby and future testing runtime.

Agreement on this in TC meeting (14th Jan 2021)
- http://eavesdrop.openstack.org/meetings/tc/2021/tc.2021-01-14-15.00.log.html#l-226

Change-Id: Ia666cc9e306bbea513e94342e92c45ba4d509d42

reference/project-testing-interface.rst

@@ -117,7 +117,6 @@ used to deploy OpenStack on:
 
 - `Latest Ubuntu LTS <https://wiki.ubuntu.com/Releases>`_
 - `Latest CentOS Major <https://www.centos.org/download/>`_
-- `Latest openSUSE Leap <https://software.opensuse.org/>`_
 
 Below are the two upstream testing required as minimum to consider the above distro for defining
 the testing runtime:

reference/runtimes/wallaby.rst

@@ -7,7 +7,6 @@ distribution <pti-linux-distros>` versions are:
 
 * Ubuntu 20.04
 * CentOS 8
-* openSUSE Leap 15
 
 Python Runtimes for Wallaby
 ============================
@@ -21,7 +20,7 @@ when all projects have migrated to a later version.
 Based on the criteria above, all Python-based projects must target and test
 against, at a minimum:
 
-* Python 3.6 (default in CentOS 8.0 and OpenSUSE Leap 15.2)
+* Python 3.6 (default in CentOS 8.0)
 * Python 3.8 (latest available; default in Ubuntu 20.04)
 
 More details on Python requirements can be found in :ref:`pti-python`.