Don't set the dns managers

Revert to previous behavior, pick up whatever is set as default
in nova. This was causing an issue in a tempest run. Tempest has
a test where multiple servers can be spun up with the same name
this test failed.

Change-Id: Ie71eda94caf38db0489d6b2385dc80808a39864d
diff --git a/stack.sh b/stack.sh
index 700b520..05f5f35 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1482,8 +1482,6 @@
         fi
     elif is_service_enabled n-net; then
         add_nova_opt "network_manager=nova.network.manager.$NET_MAN"
-        add_nova_opt "instance_dns_manager=nova.network.minidns.MiniDNS"
-        add_nova_opt "floating_ip_dns_manager=nova.network.minidns.MiniDNS"
         add_nova_opt "public_interface=$PUBLIC_INTERFACE"
         add_nova_opt "vlan_interface=$VLAN_INTERFACE"
         add_nova_opt "flat_network_bridge=$FLAT_NETWORK_BRIDGE"