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.

This is needed to fix this issue in the grenade case.

Change-Id: Ief9deeb3349a07e22fb365e58f3368c847f845ef
(cherry picked from commit 40f4438f9baa945e451f8a9fc682efe3fdf9b8f7)