Add environment variable overrides for file loaded properties

[svdberg]

We internally use the functionality of overriding loaded properties with prefixed environment variables. We convert the names of these env variables to property keys with a simple replacement scheme. This PR implements this logic. Feel free to do anything with it you like, just sharing in case there is any interest in this approach.

Example: thrift.client=proxy can be overridden with the PREFIX_THRIFT_CLIENT environment variable, where PREFIX is configurable.