Update Horizon Neutron-related settings

- Horizon no longer has "enable_security_group" setting
  so we need to remove it.
- There is no need to set enable_lb/firewall/vpn to True
  when q-lbaas/q-fwaas/q-vpn is enabled because Horizon now checks if
  Neutron ext-list and enables corresponding dashboards accordingly.

Change-Id: I37073d73e4cba0103ab1a3d935302f1cd0ef73c5
Merge "Update Horizon Neutron-related settings"