WebSocketTransport: allow to set listen address in config (fixes kurento/bugtracker#518 and kurento/bugtracker#290)

[Craeckie]

What is the current behavior you want to change?

Resolves Kurento/bugtracker#518 and Kurento/bugtracker#290 to allow the server admin to specify a listen address for the websocket server. This way the port doesn't have to be exposed to the internet.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature / enhancement (non-breaking change which improves the project)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• My change requires a change to the documentation
• My change requires a change in other repository

Checklist

• I have read the Contribution Guidelines
• I have added an explanation of what the changes do and why they should be included
• I have written new tests for the changes, as applicable, and have successfully run them locally

[j1elo]

Thank you very much! This is a nice addition for people that need or want to customize the WebSocket listening port