Add env variables for redis session storage

[raupie]

This add's two new env variables for ezplatform's generic.php file to use a redis service for session handling.

[Plopix]

Thx @raupie I will merge when the code will be merged in eZ Platform https://github.com/ezsystems/ezplatform/pull/289

Also this PR misses 2 things here these 2 news variables should be added:

• https://github.com/ezsystems/launchpad/blob/master/src/Core/DockerCompose.php#L116
• https://github.com/ezsystems/launchpad/blob/master/src/Core/DockerCompose.php#L157

Thank you!

[raupie]

variables added to core env functions.

[raupie]

How do we proceed with this PR with version 2.2 addressing it?