Prior to this change if the `bwcTest` task is run then it would create
task for each version, but each task in reality would use wireCompatVersions - 1
ES version. So we were not actually testing against 5.6.x versions in the
6.x and 6.0 branches.
This commit adds a gradle project, set inside the root build.gradle,
which controls all our bwc tests. This allows for seamless (ie no errant
CI failures) backporting of behavior.
This commit renames the backwards-5.0 qa test to mixed-cluster and
creates a test within the project per wire compat version. Like with
rolling upgrade tests, the integTest task will run against the most
recent version, while all versions will be tested with the bwcTest task.
Martijn van Groningen
Ryan Ernst