cassandra-maven-plugin: CMP-11. Allow a 'skip' option to be set from the pom file.

issue: https://jira.kiji.org/browse/CMP-11
review: https://review.kiji.org/r/2102
commit: https://github.com/kijiproject/cassandra-maven-plugin/commit/83dbed83ce9ad3f3101fe2db8cee37f49b1d8fad