Profiling OpenStack from the Developer Dashboard

Summary
=======
To provide Horizon operators, developers and cloud architects a new tool for profiling OpenStack deployment right from the Horizon Web UI.

Motivation
=========
If you have ever run Horizon Dashboard on top of large OpenStack deployment, you must have asked yourself: Why is Horizon so slow? Or is it OpenStack so slow? Identifying the real performance bottlenecks in an actual OpenStack deployment is the first step before fixing these performance issues. There already exists openstack/osprofiler Oslo library which is being integrated into different OpenStack projects (say, Nova) or is already integrated (say, Cinder). Yet profiler UI is pretty scarce and limited to CLI of osprofiler client.

To make profiling of OpenStack more accessible and thus draw a greater attention to the problems of OpenStack performance, creating a Web UI-based tool is a natural choice. Implementing it right inside Horizon also solves different Horizon performance problems as well. Moreover, integrating the osprofiler library into Horizon enables us to add a new Rally benchmarking job for Horizon, which will tell the community in a quantifiable way whether the commit under question makes Horizon performance better or worse.

Description
=========
1. Integrate osprofiler into Horizon Django middleware classes pipeline
2. Add trace points into openstack_dashboard.api wrappers
3. Implement a new panel in Developer Dashboard to allow for profiling from UI.

UX
===
A new Profiler panel appears within Developer dashboard. When profiling is enabled there, a new dropdown appears in the right side of the header bar, which allows to profile the rendering of the current page. Once it's done, a message box with a link appears which leads to a Profiler panel. There one will be able to see all the traces written on behalf of the current user, including the new one.

Wireframes, Mocks, Videos and UI Markup
---------------------------------------------------------------
N/A

Testing
======
N/A as a developer feature

Outside Dependencies
==================
MongoDB cluster needs to be installed on Devstack host (default configuration), see https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/#install-mongodb-community-edition for instructions. Then, change net:bindIp: key to 0.0.0.0 inside /etc/mongod.conf and invoke `sudo service mongod restart` for the changes to have an effect.

Requirements Update Required
========================
osprofiler and pymongo requirements are added to horizon

Doc Impact
=========
N/A