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/apts/nova b/files/apts/nova
index 38c99c7..e779849 100644
--- a/files/apts/nova
+++ b/files/apts/nova
@@ -1,5 +1,6 @@
 dnsmasq-base
 dnsmasq-utils # for dhcp_release
+conntrack
 kpartx
 parted
 iputils-arping