Improve PasswordAuthenticator default super user setup

patch by Aleksey Yeschenko; reviewed by Jeremiah Jordan for
CASSANDRA-7788
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/auth/PasswordAuthenticator.java
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge branch 'cassandra-2.1.0' into cassandra-2.1