ACCUMULO-2379 Document running functional tests under MR as a different user

Functional tests might not run under MapReduce in clusters where a different user from
the Accumulo user runs mappers. Needed configuration changes are now documented in
the functional test README.
Merge branch '1.4.5-SNAPSHOT' into 1.5.1-SNAPSHOT

Conflicts:
	test/system/auto/README
Merge branch '1.5.1-SNAPSHOT' into 1.6.0-SNAPSHOT