add needsFlowServer to flowCmd test step
add needsFlowServer to flowCmd test step

Summary: lets us tell the test harness not to automatically start a flow server when doing a `flowCmd` step when we don't want it to

Reviewed By: mroch

Differential Revision: D31282538

fbshipit-source-id: a5e525d97625274d9ef675ff03b18165becad5f8

facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow