OPENSTACK_NEUTRON_NETWORK no longer supports the following settings[1].
- enable_firewall
- enable_lb
- enable_vpn
This change removes the remanining usage of these settings from
installation guide.
[1] 53a5164d82a7412d10cee634cd0755336f566e24
Change-Id: I9c75bafc2301f045096b51f433456d1accb12edf