ACCUMULO-2042 Get scalability test running

Two problems prevented scalability tests from being run properly.
1. The Run class was looking for test classes under the "accumulo"
   package instead of "org.apache.accumulo".
2. A results subdirectory on the local filesystem was not being
   created, and so result data from HDFS could not be copied out.

This commit also updates the scalability test README with more
detail.