Amend reference/PTI to add openSUSE

When the Ubuntu and CentOS were added in the PTI as "supported
distros", SUSE was already one of the most-popular distros [1],
and therefore deserves listing here.

I don't believe the PTI should refer to "popular" term for distros,
as this isn't a popularity contest. Instead, we should allow any
distribution where community effort is provided.

In this case, openSUSE images are present in infra, and SUSE contributors
are using devstack on openSUSE. Next to that, deployment projects like
openstack-ansible or openstack-helm are making use of openSUSE.

Last, but not least, some projects already take into account openSUSE
in their regular work (for example, when the nova team contacts the
distributions at the beginning of the cycle to know their latest
supported libvirt [2]).

I believe all these arguments are enough to warrant the addition of
openSUSE in the list of distributions.

[1]: SUSE was ranked 4th in that survey (with SLES), after RHEL (3rd).
[2]: http://lists.openstack.org/pipermail/openstack-dev/2018-September/135007.html

Change-Id: I1ac3b7477b5d4cb4af75c319b4232d20b4b4b284

reference/project-testing-interface.rst

@@ -112,10 +112,11 @@ Language specific instructions supplement these and are in addition to them.
 Linux Distributions
 -------------------
 
-The following operating systems are the most popular when deploying OpenStack:
+The following operating systems are regularly used when deploying OpenStack:
 
-- Ubuntu (`latest LTS`_)
-- CentOS (`latest stable`_)
+- Latest Ubuntu LTS (`latest LTS`_)
+- Latest CentOS Major (`latest stable`_)
+- Latest openSUSE Leap (`latest Leap release`_)
 
   .. note::
 
@@ -137,6 +138,7 @@ tests might have to be skipped on these distributions.
 
 .. _latest LTS: https://wiki.ubuntu.com/Releases
 .. _latest stable: https://www.centos.org/download/
+.. _latest Leap release: https://software.opensuse.org/
 
 .. toctree::
    :maxdepth: 1