Rename dns_server, add var for selinux. (#1572)

* Rename dns_server to dnsmasq_dns_server so that it includes role prefix
as the var name is generic and conflicts when integrating with existing ansible automation.
*  Enable selinux state to be configurable with new var preinstall_selinux_state

roles/docker/tasks/set_facts_dns.yml

@@ -6,7 +6,7 @@
       {%- if dns_mode == 'kubedns' -%}
         {{ [ skydns_server ] }}
       {%- elif dns_mode == 'dnsmasq_kubedns' -%}
-        {{ [ dns_server ] }}
+        {{ [ dnsmasq_dns_server ] }}
       {%- endif -%}
 
 - name: set base docker dns facts