Update proxy behavior

[markdboyd]

Changes proposed in this pull request:

Related to https://github.com/cloud-gov/private/issues/2015

• Update proxy to set environment variables for the Redis backend used for session data

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

Nothing about the actual security of how the proxy authenticates users is changing, the storage of the session data is just changing from a cookie in the browser to a cookie in the browser that references data stored in a backend. It is theoretically more secure for actual session data to reside in a backend that cannot be intercepted at the browser level.

[JasonTheMain]

Looks great to me