Convert OS specific packages to new format

Uses the logic introduced in the previous patch to convert all
kubernetes/preinstall/vars/* os specific files to the `pkgs`
dictionary.

Some niceties for devs:
- always validate the `pkgs` variable to catch mistakes in CI.
- ensure that `pkgs` is always sorted. This makes it easier to find the
  packages you're looking for.

roles/kubernetes/preinstall/tasks/0020-set_facts.yml

@@ -199,20 +199,6 @@
       supersede domain-name-servers {{ (nameservers | d([]) + cloud_resolver | d([])) | unique | join(', ') }};
   when: dns_early and not dns_late
 
-- name: Gather os specific variables
-  include_vars: "{{ item }}"
-  with_first_found:
-    - files:
-        - "{{ ansible_distribution | lower }}-{{ ansible_distribution_version | lower | replace('/', '_') }}.yml"
-        - "{{ ansible_distribution | lower }}-{{ ansible_distribution_release }}.yml"
-        - "{{ ansible_distribution | lower }}-{{ ansible_distribution_major_version | lower | replace('/', '_') }}.yml"
-        - "{{ ansible_distribution | lower }}.yml"
-        - "{{ ansible_os_family | lower }}.yml"
-        - defaults.yml
-      paths:
-        - ../vars
-      skip: true
-
 - name: Set etcd vars if using kubeadm mode
   set_fact:
     etcd_cert_dir: "{{ kube_cert_dir }}"