docker-based integration tests

[tpetr]

This PR further docker-ifies Singularity. Using the docker-maven-plugin, it:

• Builds a SingularityService docker image after packaging
• Adds a SingularityServiceIntegrationTests module, which uses the docker plugin to stand up a Mesos cluster with zk, a master, 3 slaves, and the most recently built Singularity.

Other things to note:

• Docker-related stuff will only be activated if the DOCKER_HOST env var is set.
• docker-maven-plugin requires Maven 3.2.1+