Generate graylog2-elasticsearch.conf From Env Variables

This enables the capability to generate the graylog2-elasticsearch.conf file from environment variables that are prefixed with ``GEC_`` (**G**raylog2 **E**lasticsearch **C**onfig). :-)

Added python to the mix to get the deed done. With python present it might be a good idea to simply refactor the generate-configs.sh logic into the python file.