Facilitate full incoming webrtc load testing logic

[atsakiridis]

Up to now we have been testing outgoing webrtc load testing logic (MS -> webrtc clients). With this issue we'll fully implement incoming load testing logic. This is how it will work:

• restcomm-test will spawn the node js http server but instruct it to serve different RCML that will not make outgoing webrtc call, but instead connect to some sort of Say App or sth similar directly (the http-server.js script will accept the '--client-role' option that will govern that and default to 'passive')
• the web app should be configurable to work in either passive (receiving calls) or active mode (making calls) : already implemented at Enhance load testing tools to be able to initiate webrtc calls towards restcomm (using registrarless) #68
• restcomm-test will continue to fire up the 're-spawn' server so that when the web app finishes calling Restcomm and closing the tab, it should be able to notify the script to spawn a new tab. But will also need to convey to http-server.js the type of client role, based on its own --client-role