Enable associating routers in a tenant with firewall resource

Summary
=======
The goal of this blueprint is to extend Firewalls configuration within
Horizon, during creation and edit to allow associating routers in the tenant
with the Firewall resource and update the Firewall Details page to show
associated router(s) information.

Motivation
==========
A neutron blueprint fwaas-router-insertion has been proposed and approved in
kilo that allows associating routers to Firewall. This blueprint is to allow
user to specify routers from Horizon when creating / updating Firewall to
support the implementation of fwaas-router-insertion blueprint.

Description
===========
Network/Firewalls/Firewalls will be extended when FWaaS plugin is enabled.

When creating a firewall, the "Add Firewall" page will be extended to allow
user to specify a router.

When displaying details of a firewall that has been created, the associated
router(s) Name and ID will be displayed.

When updating a firewall, the "Edit Firewall" page will be extended to
allow an user to associate additional routers or dis-associate existing
routers.

UX
==
When user clicks "+Create Firewall" under Network/Firewalls/Firewalls, "Add
Firewall" page will pop-up. This page will be extended to allow user to
specify none, one or multiple "Router ID(s)".

Under Network/Firewalls/Firewalls page, when user clicks a Firewall (name)
that has been configured, the "Firewall Details" page will be extended to
display associated Router Name(s) and ID(s), if any.

Under Network/Firewalls/Firewalls page, when user clicks "Edit Firewall" on
a firewall that has been configured, the "Edit Firewall" page will be shown
to allow user to update firewall. This page will be extended to allow user
to associate additional routers or dis-associate existing routers.

Wireframes, Mocks, Videos and UI Markup
---------------------------------------------------------
Will post these up as code is developed.

Testing
=======
This blueprint will be testable through standard devstack deployments with
Neutron and FWaaS enabled.

Outside Dependencies
====================
https://blueprints.launchpad.net/neutron/+spec/fwaas-router-insertion

Requirements Update Required
============================
No requirements changes to the Dashboard

Doc Impact
==========
No changes default behaviors, settings, or deprecation of previous code.