Build does not publish signatures to maven

We have been using the maven-publish plugin in our build to publish build artifacts to maven repositories. Unfortunately this has some limitations when used for release builds as it is incapable of signing poms or uploaded signature files (see https://issues.gradle.org/browse/GRADLE-3183).

We should use the nexus plugin instead, see the samza approach at https://github.com/apache/samza/blob/master/gradle/release.gradle.