ansible: add dnspython dependency

This dependency is required to use the lookup plugging for performing
dns lookups.

http://docs.ansible.com/ansible/playbooks_lookups.html#the-dns-lookup-dig

pkgs/top-level/python-packages.nix

@@ -877,7 +877,7 @@ in modules // {
 
     propagatedBuildInputs = with self; [
       pycrypto paramiko jinja2 pyyaml httplib2 boto six
-      netaddr
+      netaddr dns
     ] ++ optional windowsSupport pywinrm;
 
     meta = {
@@ -914,7 +914,7 @@ in modules // {
 
     propagatedBuildInputs = with self; [
       pycrypto paramiko jinja2 pyyaml httplib2 boto six readline
-      netaddr
+      netaddr dns
     ] ++ optional windowsSupport pywinrm;
 
     meta = with stdenv.lib; {