Layered config files

As explained in the README, I'd like to automate benchmarking which means adding a new 'version' section to the config. To automate this at the moment I'd need to write a whole new config file or use something ugly like sed to replace the version with the one under test. Instead, this change allows several config files to be supplied, so I can overlay one file containing just a buildstream version on top of another fixed set of test and volume configuration.