There seems to be issues with the write handling of the osmo-stp
and so far we did not enable vty tests here. Make it possible to
enable the tests, fix the VTY strings, change string to OsmoSTP.
Change-Id: I547fd4840d86ce16e8589fb63802dd7099781194
Create a simple queue for pending DTMF tones, play them using the
MTN API, and then send the next tones once the playback is complete.
The callback and scheduling is done from the same context so no locking
needs to be done.