Allow overriding cassandra-rackdc.properties location

Patch by Patrick Ziegler, reviewed by brandonwilliams for CASSANDRA-7072
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/locator/SnitchProperties.java
Merge branch 'cassandra-2.1' into trunk