Feature/qeng 969 add modify tk config dsl method

JVM Puppet for one would choke on an empty config, but if the tester wants to do that then I would suggest creating an empty file using `create_remote_file` instead of `modify_tk_config`. 

This method should only modify an existing config or replace its contents entirely with a string generated from a non-empty hash. I can imagine someone passing in a hash they don't know the contents of (for example one obtained as a result of some intermediate operation), expecting it to only merge values into the file, and be caught unawares when they get an empty file.