Add cqlsh completion for IF [NOT] EXISTS in CREATE/DROP USER

Patch by Michał Michalski; review by Tyler Hobbs for CASSANDRA-7611
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Add cqlsh completion for IF [NOT] EXISTS in CREATE/DROP USER

Patch by Michał Michalski; review by Tyler Hobbs for CASSANDRA-7611

Conflicts:
	CHANGES.txt