Clarify supported distro versions

This role only support openSUSE Leap 15, not 42.3, not tumbleweed.
Saying all is confusing and a bad practice. This fixes it, while
ensuring the job is properly defined to test that assertion.

At the same time, this lists the CentOS support and updates the
ubuntu supported versions.

Change-Id: I8fd01b9184aa1471bb0a7f950cc802ceca57f506

meta/main.yml

@@ -30,9 +30,13 @@ galaxy_info:
     - stretch
   - name: Ubuntu
     versions:
-    - trusty
-    - xenial
     - bionic
+  - name: EL
+    versions:
+    - 7
+  - name: opensuse
+    versions:
+    - 15
   galaxy_tags:
     - cloud
     - openstack