[terraform] include REDIS_URL as part of keyserver terraform config

Summary:
This is necessary for the keyserver to access our redis instance. We use rediss:// as we have encryption in transit enabled for our redis instance, meaning
we need to access the instance with tls enabled

Depends on D12690

Test Plan:
Was able to launch two devices which connected to my self hosted keyserver and sent real-time messages to each other

Without specifying REDIS_URL, keyserver nodes would fail. Using redis:// instead of redis:// didn't allow for real-time communication.

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12691