VROUTER - DNS default variables

If user doesn't override the 
```
        ONEAPP_VNF_DNS_ENABLED = "YES",
        ONEAPP_VNF_DNS_NAMESERVERS = "1.1.1.1,8.8.8.8",
        ONEAPP_VNF_DNS_USE_ROOTSERVERS = "NO",
```
parameters on the CONTEXT, take the defaults from the NIC for instance.