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: Ie84846f8590c3ae60afd671d973a4de3c644d526

meta/main.yml

@@ -23,7 +23,7 @@ galaxy_info:
   platforms:
     - name: Debian
       versions:
-        - stretch
+        - buster
     - name: Ubuntu
       versions:
         - bionic

tasks/tempest_install.yml

@@ -18,7 +18,7 @@
   package:
     name: "{{ tempest_package_list }}"
     state: "{{ tempest_package_state }}"
-    update_cache: "{{ (ansible_facts['pkg_mgr'] in ['apt', 'zypper']) | ternary('yes', omit) }}"
+    update_cache: "{{ (ansible_facts['pkg_mgr'] == 'apt') | ternary('yes', omit) }}"
     cache_valid_time: "{{ (ansible_facts['pkg_mgr'] == 'apt') | ternary(cache_timeout, omit) }}"
   register: install_packages
   until: install_packages is success