Remove references to unsupported operating systems

All references to Gentoo, SUSE, Debian stretch and Centos-7  are removed.
Conditional tasks, ternary operators and variables are simplified where possible
OS specific variables files are generalised where possible

Change-Id: Ia09789da8c8137c982dcd7560fe61a98a07309e7

meta/main.yml

@@ -22,14 +22,11 @@ galaxy_info:
   platforms:
     - name: Debian
       versions:
-        - stretch
+        - buster
     - name: Ubuntu
       versions:
-        - xenial
         - bionic
-    - name: opensuse
-      versions:
-        - 15
+        - focal
   galaxy_tags: []
 dependencies:
   - role: apt_package_pinning

vars/suse.yml

@@ -1,16 +0,0 @@
----
-# Copyright 2018, SUSE Linux GmbH.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-rally_distro_packages: []