Fix availability validation for LOCAL_ONE CL

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-7319
Merge branch 'cassandra-1.2' into cassandra-2.0
Merge branch 'cassandra-2.0' into cassandra-2.1

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