ACCUMULO-1893 keep tests from attempting to write to the root of HDFS.

Reapplication of part of ACCUMULO-1563 to current branch.

Author: Jon Hsieh <jmhsieh@apache.org>

Signed-off-by: Keith Turner <kturner@apache.org>
Merge remote-tracking branch 'origin/1.4.5-SNAPSHOT' into 1.5.1-SNAPSHOT

Conflicts:
	test/src/main/java/org/apache/accumulo/test/CreateRFiles.java
	test/src/main/java/org/apache/accumulo/test/functional/BulkSplitOptimizationTest.java