Prefix non-standardised platform keys

At the London Build Meetup @sstriker suggested to prefix all platform keys with the backend type, e.g. `rbe-container-image` instead of `container-image` (enforce namespaces).

Sander, can you please remind me the rationale for this?

No files or tests are named. First locate the definitions and uses of platform keys, then determine the rationale and scope for prefixing them with the backend type. Done requires an agreed naming scheme and consistent handling of the affected keys.

Written by the indexing model from the issue text.