Configuration['results_directory'] should be used everywhere

[josephharrington]

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.