Include message of the day option

Summary
=======

Provide the ability for operators to configure message of the day option.

Motivation
========

As an operator there are many cases where I would like to notify my users about something through a message of the day. Things such as:

* We just upgraded to a new version of X, and this is what changed
* Planned outages
* etc.

This idea came up in the ops summit in Paris, where one operator said he doesn't upgrade Horizon because people don't like surprise changes.

A couple of operators have also been asking about this.

Description
=========

Add a local_settings called MESSAGES_PATH in horizon to allow operators to specify the directory where to lookup messages that will be displayed to the user.

The directory may contain one or more message file, horizon will display all the messages in the directory. Each message file contain a JSON formatted data:

{"level": "info", "message": "motd message"}

The available value for level is info, success, warning and error

Horizon will display the message to the user based on the level set in the message file.

Example configuration:

MESSAGES_PATH = /motds/

Files under: /motds
- 001_motd.msg
contains: {"level": "info", "message": "motd message 1"}
- 002_motd.msg
{"level": "warning", "message": "motd message 2"}

Expected:
When the user login, two message will pop-up
1. "Success: motd message 1"
2. "Warning: motd message 2"

UX
===
NA

Additional notes can be found: https://bugs.launchpad.net/horizon/+bug/1392855