Configuration['results_directory'] should be used everywhere

Right now the master always uses `Configuration['results_directory']` to store and read build results, and the slave always uses `Configuration['artifact_directory']`. These are currently set to `~/.clusterrunner/results/master` and `~/.clusterrunner/artifacts` respectively.

There are a few annoying things about this:
- In this context there's no difference between "results" and "artifacts". These two directories hold the same files (e.g., timing data, console output, anything the user put into $ARTIFACT_DIR). The only difference is that one is used on the master and one is used on the slave. It's really hard to remember which goes with which.
- Having two separate conf settings complicates the code, and requires us to pass directory parameters into some of the code common between master and slave (e.g., BuildArtifact, ConsoleOutput both have a `result_root` parameter for this reason).
- `Configuration['results_directory']` is unused on the slave (but it is currently set to `~/.clusterrunner/results/slave`).

I think we should get rid of `Configuration['artifact_directory']` and just use `Configuration['results_directory']` everywhere. This value already resolves to a different, more intuitive directory on the slave and would leverage configuration to reduce the number of parameters passed into common code.