Add the ability to theme sandboxes

Created by: eeue56

The problem

As defined here, this PR theme variables to an instance.

The solution

In order to make this work, I made the data passed down compatible with existing playbooks such as the one found here.

The changes

The new instance variables added are:

• theme_name (defaults to 'default')
• theme_source_repo (defaults to 'https://github.com/Stanford-Online/edx-theme.git')
• theme_version (defaults to 'master')

To change the default options, you can set the following variables in your .env file appropriately:

• THEME_NAME
• THEME_SOURCE_REPO
• THEME_VERSION

These settings are then sent to ansible as the following:

edxapp_theme_name: '{{ instance.theme_name }}'
edxapp_theme_source_repo: '{{ instance.theme_source_repo }}'
edxapp_theme_version: '{{ instance.theme_version }}'

I've added some tests to make sure ansible gets populated correctly, and that the defaults are as expected.

Possible future work

It might be sensible to provide some more documentation on the properties in .env. AFAIK there's nothing other than what's provided in the default .env examples.

@antoviaque, what do you think? Is there anything I haven't covered?