Use environment variables to pass keys and Silk server url

[luigiselmi]

Keys to access the social networks should be passed via environment variables instead of from a file. This solves two issues, security and instantiation of a docker container.

[luigiselmi]

Another way to pass the keys using the conf/application.conf file is to mount a volume, when starting a container with Fuhsen, that points to a different copy of application.conf from that available on Github, that contains the keys.