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: Ibec329fa5dca39222519b99ad27d78b46c11633e

meta/main.yml

@@ -9,14 +9,14 @@ galaxy_info:
   platforms:
   - name: Debian
     versions:
-    - stretch
+    - buster
   - name: Ubuntu
     versions:
-    - xenial
-    - bionic
+      - bionic
+      - focal
   - name: EL
     versions:
-    - 7
+      - 8
   galaxy_tags:
    - cloud
    - openstack

vars/suse.yml

@@ -1,21 +0,0 @@
----
-# Copyright 2018
-#
-# 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.
-#
-# (c) 2018 Jonathan Rosser <jonathan.rosser@rd.bbc.co.uk>
-cache_timeout: 600
-
-trove_distro_packages:
-  - systemd-devel
-  - qemu-tools