basertpendpoint: add externalIPv4 and externalIPv6 parameters

[dpocock]

What is the current behavior you want to change?

RTP endpoint currently selects the media IP address in a random manner.

On a multi-homed host this can be problematic.

What is the new behavior provided by this change?

Adding configuration parameters externalIPv4 and externalIPv6 in BaseRtpEndpoint.conf.ini

How has this been tested?

Testing with my reConServer / reSIProcate / Kurento integration branch

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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 https://github.com/Kurento/kms-core/pull/28

Checklist

• [x] I have read the Contribution Guidelines
• [x] 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