Allow session timeout to be configurable

[robvdl]

It does have a session timeout. I've seen pyramid-session-redis time out sessions all the time during development.

It would be nice to make this configurable.

pyramid-session-redis does support TTL on session keys so they auto delete even if the app is down, I'm pretty sure that is what the default setup is using. But it would be nice to be abble to configure this with an environment variable.