Optional field update for DNS template

[ehsandeep]

Please describe your feature request:

Currently, DNS block contains name,type,class,recursion,retries as mandatory fields to include in the template, where
only the type is subject to change most of times, as such we can make last three fields optional with use of sane default values.

Current default:-

dns:
  - name: "{{FQDN}}"
    type: NS
    class: inet
    recursion: true
    retries: 3

After update default:

dns:
  - name: "{{FQDN}}"
    type: NS