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: I705b1ea8907a12d30a05a7b3ebbbfb7a8a021325

meta/main.yml

@@ -20,16 +20,16 @@ galaxy_info:
   license: Apache2
   min_ansible_version: 2.3.0
   platforms:
+    - name: Debian
+      versions:
+        - buster
     - name: Ubuntu
       versions:
-        - xenial
         - bionic
+        - focal
     - name: EL
       versions:
-        - 7
-    - name: opensuse
-      versions:
-        - all
+        - 8
   galaxy_tags:
     - cloud
     - rabbitmq