Add conntrack-tools to nova dependencies

'Error deleting conntrack entries' appears in all
n-net. Adding conntrack-tools/conntrack as nova dependency,
in order to eliminate this issue.

Change-Id: Ief9deeb3349a07e22fb365e58f3368c847f845ef
diff --git a/files/rpms/nova b/files/rpms/nova
index e05d0d7..fa472a8 100644
--- a/files/rpms/nova
+++ b/files/rpms/nova
@@ -1,6 +1,7 @@
 MySQL-python
 curl
 dnsmasq-utils # for dhcp_release
+conntrack-tools
 ebtables
 gawk
 genisoimage # required for config_drive