Implement config override using environment variables or system properties

Each configuration setting for graylog2-server and graylog2-radio can now be overridden using an environment variable or a system property.

Environment variables need to be uppercase and start with the prefix "GRAYLOG2_" (e. g. "GRAYLOG2_IS_MASTER").
System properties need to start with the prefix "graylog2." (e. g. "graylog2.is_master").

Fixes #639