Horizon plugin support for Quotas and Service Info

Summary
=======
The goal of this blueprint is to make the quotas form in Project page and default quota page pluggable and allow horizon plugins to register their quota resources.

Motivation
========
Currently, Quota form in Project page, default quotas and service info contains only compute/volume/network information, but it is nice if these pages support horizon plugin resources. This blueprint aims to allow horizon plugins to add quotas and service info tabs.

Description
=========
The following quota related UI will be touched.
Table and/or form will be tabified per service and then they are pluggable for horizon plugins.

* Admin->System->Defaults panel
  * Default quotas table
  * Update Defaults form
* Identity->Projects
  * Modify Quotas action
  * Create Project action ("Quotas" tab will be dropped from UX perspective)

Service information tabs in Admin->System->System Information are also changed to be pluggable.

The pluggable mechanism is achieved via python entry points.
Each horizon plugins can define an entry point in its setup.cfg like below.

 [entry_points]
 openstack_dashboard.admin.default_quota_tabs =
   orchestration = <tab class name>

Entry point names for individual tabs will be:

* openstack_dashboard.admin.defaults.default_quota_table
* openstack_dashboard.admin.defaults.update_default_quota_form
* openstack_dashboard.identity.projects.update_quotas
* openstack_dashboard.admin.info.service_table

UX
===
Tab will be introduced into the default quota table and the service table in the System information panel.
Workflow tab will be introduced into the update default quota form and the update project quota form.

"Quotas" tab will be dropped from the "Create Project" form.
After tabifying the quota form, it is not a good idea to have multiple tabs related to quotas in "Create Project" form.
I don't think this impacts a lot from UX perspective. An operator can modify project quotas just after creating a new project.

Testing
=======
All existing features including default quotas table, updating default quotas, service list table and updating project quotas should work.

Outside Dependencies
==================
No outside dependencies.

Requirements Update Required
========================
Nothing.

Doc Impact
=========
Horizon plugin documentation can be updated.