cassandra-maven-plugin: CMP-3. Cassandra Maven plugin should delete existing target/cassandra-it directory before running.

issue: https://jira.kiji.org/browse/CMP-3
review: https://review.kiji.org/r/2046
commit: https://github.com/kijiproject/cassandra-maven-plugin/commit/3c15d0fa78405c5767b76eacb2b62291d0a4b2de