externalize integration test set up as seperate module

[meshpaul]

Hi I think as minor enhancement and generalization the integration test set up code can be externalized as separate module. Your work can be re-used for integration testing for thouse who want to utilize this. Since this is docker specific and some CI tools like concourse may have an issue with this perhaps name this as such vault-java-driver-test-harness-docker or similar what do you think?