Add selection of Django session engine to this module

This allows Django to be set to use something other than
the default session engine through Puppet. By default,
it will have no impact on existing configurations.

We've added this because our security constraints make signed cookies
as a session store for Horizon undesirable. The reasons for this are:

  * Sessions become vulnerable to replay attacks,
    a clean log-out is difficult to achieve.
  * Serializing sessions in cookies may allow an attacker
    to escalate read-only access to server configuration
    into code execution via deserialization of attacker-generated
    content.

Change-Id: Iaf80bdc1bb9ce55ff46b2dcfac73daad90fd0d37
Merge "Add selection of Django session engine to this module"