kiji-poms: POM-36. Manage exec and build-helper Maven plugins through root pom.

issue: https://jira.kiji.org/browse/POM-36
review: https://review.kiji.org/r/1919
commit: https://github.com/kijiproject/kiji-poms/commit/5e17056da9dc7c76cea0eeffa26e93a1c734a59d