coturn: improve telnet connection workflow

[kris-lab]

Part of https://github.com/cargomedia/puppet-cargomedia/issues/1482

[kris-lab]

@njam I have added waiting for prompt logic. I assume we send command too quick which seems to be truth based on errors are thrown.

Hard to add any sensible spec. I suggest to test/review this manually.

[njam]

lgtm!