kubernetes/preinstall: dns setting cleanup(dhclient, resolvconf)

We use a lot of facts where variables are enough, and format too early,
which prevent reusing the variables in different contexts.

- Moves set_fact variables to the vars directory, remove unnecessary
 intermediate variables, and render them at usage sites to only do logic
 on native Ansible/Jinja lists.
- Use defaults/ rather than default filters for several variables.

roles/container-engine/docker/tasks/set_facts_dns.yml

@@ -10,12 +10,12 @@
 
 - name: Add upstream dns servers
   set_fact:
-    docker_dns_servers: "{{ docker_dns_servers + upstream_dns_servers | default([]) }}"
+    docker_dns_servers: "{{ docker_dns_servers + upstream_dns_servers }}"
   when: dns_mode in ['coredns', 'coredns_dual']
 
 - name: Add global searchdomains
   set_fact:
-    docker_dns_search_domains: "{{ docker_dns_search_domains + searchdomains | default([]) }}"
+    docker_dns_search_domains: "{{ docker_dns_search_domains + searchdomains }}"
 
 - name: Check system nameservers
   shell: set -o pipefail && grep "^nameserver" /etc/resolv.conf | sed -r 's/^nameserver\s*([^#\s]+)\s*(#.*)?/\1/'