Hide L3 router panel by configuration

The goal of this blueprint is to allow operators to disable L3 Router features by configuration options.

Neutron setup allows operators not to enable Router feautre (by not using Router service plugin in neutron.conf) [bug 1292022],
and some Neutron plugin does not support Router feature [bug 1160580].

According to bug 1350639 there is also a case where operators want to disable Router panel from their policy.

Considering the above, it seems nice we have a option to control Router panel.

The proposed solutions are:

(1) Add a new config option 'enable_router' to OPENSTACK_NEUTRON_NETWORK.
The default is True as router feature is enabled in most deployments and it is the current default behavior of Horizon.
If this option is False, Router panel will disappear.

(2) Network Topology panel shows routers in the topology map and also has "Create Router" button.
If "enable_router" set to False, routers in the topology map are not displayed now and "Create Router" button is not shown.

(3) Introduce "enable_floatingip" option to OPENSTACK_NEUTRON_NETWORK.
In Neutron, floating IP feature is provided by L3 router extension, so Floating IP feature also needs to be disabled in deployments without L3 router feature. On the other hand, there may be a case where operators just want not to provide router features to users. To handle this case, I think it is nice to have a separate option for "floating_ip".
This option only affects Neutron enabled setup. If this option is set to False, "Floating IP" tab and "Associate/Disassociate Floating IP" menu in the instance table are not shown.