Make `CUP_PEM_KEYS` configurable

[kbence]

I'd like to deploy omaha-server using your docker image from Docker Hub (actually as an image built from that), but without forking the project. At the moment, most of the settings are configurable from environment variables but CUP_PEM_KEYS is just put into settings.py.

Since it's a dictionary, I think it could be passed as a JSON or some other arbitrary format (eg. id:path:id:path where : is os.pathsep).

I'm happy to open a PR, I'm just curious about your opinion.