Clean sbt integration test project. (#6584)
Clean sbt integration test project. (#6584)

Summary:
If there are changes on the integration tests we have to clean the
project as well since Jenkins reuses nodes.
mesosphere/marathon,mesosphere/marathon,gsantovena/marathon,mesosphere/marathon,mesosphere/marathon,mesosphere/marathon,gsantovena/marathon,gsantovena/marathon,gsantovena/marathon,gsantovena/marathon