Outbound Caller ID rules for WebRTC Demo

[marca56]

• if a DID is assigned to a username,
  ** then the DID should be sent as caller ID on calls to the PSTN
  ** then the username should be sent as caller ID on P2P calls
• if no DID is assigned to a username,
  ** then NULL (meaning empty string) should be sent as caller ID on calls to the PSTN
  ** then the username should be sent as caller ID on P2P calls

These rules work for RCML apps, but should also work for WebRTC Demo calls :(

[anhntnguyen]

https://telestax.atlassian.net/browse/RESTCOMM-1177

[ammendonca]

Issue moved to RestComm/olympus #149 via ZenHub