common: Unset default DNS servers

This seems way more correct to me as our DNS config is all over the place
and at least this way, we get correct fallback configs right off the bat.

roles/common/defaults/main.yml

@@ -2,6 +2,4 @@
 configure_network: false
 enable_zram_swap: false
 network_interface: "e*"
-dns_servers:
-  - 8.8.8.8
-  - 8.8.4.4
+dns_servers: []