Add wait-functionality for arbitrary step

[cjmckenna]

For the performance test use case, it would be nice to have a wait step that listens for the completion of the test execution before going directly on to the wait for evaluation step. In a long running test unless you set the wait time to a very high number to avoid timeout the pipeline can fail waiting for the evaluation to complete.

The process should wait to receive a test finished event from keptn before waiting for the eval to complete