dtls: Generate ECDSA certificates rather than RSA certificates

I imagine this should be made configurable, but given the way that "generate a certificate" is currently signalled (a null pem property) I'm not sure how best to signal that it should be ECDSA rather than RSA. If guidance is given I'm happy to adjust this merge request to make this configurable.

ECDSA certificates are used by Chrome by default, and are expected by BouncyCastle DTLS library used by several WebRTC server implementations. Specifically, this patch makes it possible to use gstreamer's webrtcbin to connect to Jitsi VideoBridge.