packaging: changed override-firewall in firewall-manager

In engine-setup, changed the option override-firewall that the user
may expect to be boolean into firewall-manager that accept "None",
"Firewalld" or "IPtables" as values.

Updated the output messages explaining the meaning of None as
firewall manager.

Change-Id: Ib5d92079a7caa301de248e90740e77fa5577b068
Bug-Url: https://bugzilla.redhat.com/901987
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>