Also fixes two minor bugs:
1. Change default Boskos wait timeout to 2 minutes.
This is because the current test timeout is configured to 10 minutes.
Running each test case taks 1-2 minutes, and each node will run 1-2 test
cases. 5 minutes timeout on waiting for Boskos may cause a test timeout,
which we want to avoid.
2. Create artifact subdir with 0755 rather than 0644.
Because execution bit should be set on the directories.
