add option for fixed DNS server

files/cloud_init.cfg.tftpl

@@ -49,3 +49,9 @@ fs_setup:
 mounts:
 - ["data.1", "/mnt/data"]
 %{ endif }
+%{ if fixed_dns != "" }
+manage_resolv_conf: true
+resolv_conf:
+  nameservers:
+    - "${fixed_dns}"
+%{ endif }