Switch to role management instead of user one for cassandra > 2.2

CREATE USER, DROP USER, LIST USERS are deprecated in favor of
CREATE ROLE, DROP ROLE, LIST ROLES.
This PR introduces the changes regarding user management by
switching to roles for cassandra version greater or equal 2.2

CREATE USER is supported for backwards compatibility only.
Authentication and authorization for Cassandra 2.2 and later are
based on ROLES, and use CREATE ROLE instead [1]

[1]. http://docs.datastax.com/en/cql/3.3/cql/cql_reference/cqlCreateUser.html