Add a config option to disable Router panel

Neutron setup allows operators not to enable Router feautre
(by not using Router service plugin in neutron.conf) and
some Neutron plugin does not support Router feature.
According to bug 1350639 there is also a case where operators
want to disable Router panel from their policy.
It seems nice we have a option to control Router panel.

This commit add a new option 'enable_router' to OPENSTACK_NEUTRON_NETWORK.
The default is True since 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.

blueprint hide-router-panel-by-config
Partial-Bug: #1292022
Closes-Bug: #1350639
Change-Id: If90517bd35c5b18173c54c14a1ba229f97ba4797
Merge "Add a config option to disable Router panel"