refactor Retry in tests/cmd/helper.go

Refactor [Retry](https://github.com/deis/workflow-e2e/blob/master/tests/cmd/helper.go#L65-L83) to accept `timeout` of type `time.Duration` as opposed to type `int`, potentially enabling passing expected exit code was well. Alternatively, in https://github.com/deis/workflow-e2e/pull/237 adds `RetryUntilResult` which does both, so `Retry` might be able to just invoke this function for most of its logic...