Aurora-193: fix license plugin

Adds the license header plugin and sets the correct source sets.

Replaces patch in review: https://reviews.apache.org/r/17771/

Testing Done:
All tests passed locally with rm -rf .gradle && ./build-support/jenkins/build.sh

Bugs closed: AURORA-193

Reviewed at https://reviews.apache.org/r/18316/
Adding aurora_admin sla_probe_hosts command.

Bugs closed: AURORA-209

Reviewed at https://reviews.apache.org/r/18440/