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: I4f0db0ab31e9d394d7f741aff88b53bc9129daa1

meta/main.yml

@@ -20,16 +20,16 @@ galaxy_info:
   license: Apache2
   min_ansible_version: 1.6.6
   platforms:
+    - name: Debian
+      versions:
+        - buster
     - name: Ubuntu
       versions:
         - bionic
-        - xenial
+        - focal
     - name: EL
       versions:
-        - 7
-    - name: opensuse
-      versions:
-        - all
+        - 8
   categories:
     - cloud
     - rabbitmq

tests/group_vars/hosts.yml

@@ -18,8 +18,3 @@ lxc_cache_distro_packages: "{{ _lxc_cache_distro_packages |
                                reject('match', '^openssh.*') |
                                list }}"
 lxc_cache_sshd_configuration: []
-# OpenSUSE has sshd installed and enabled by default. The other container
-# caches do not. We will just drop a systemd mask file to make sure it never
-# starts in suse.
-lxc_cache_prep_post_commands: |
-  ln -s /dev/null /etc/systemd/system/sshd.service