Add EDX_PLATFORM_REVISION env var for Studio/LMS

Update sandbox ami, fix edx and mongo repo errors

This is useful for debug output in edx-platform. We used to have the app
generate this value by doing a call to dealer.git on startup, but that
was slow.

Update Mongo 3.2 apt gpg key

veda pipeline worker

Coryleeio/install splunk app via ansible

Upgrade flower to 0.9.2 and pin celery version

Update baked_config.sh to support sandbox

The Credentials Service now relies solely on the central JWT issuer (e.g. LMS) for issuing tokens, rather than a service-specific issuer. The JWT authentication override has been removed.

LEARNER-3441

Bump jenkins instance cap for new subnet