conference.cpp

Use the negotiated medias to start conference in a correct state
(e.g. if calling a swarm in audio-only we should only see
one audio only participant). The previous code was incorrectly
starting the conference and adding an extra partipant when
attaching the call.

Note: lot of code in conference is also currently re-factored for
homogeneizing 1:1 and video conferences. So I prefered to avoid
to change a lot of things there.

A test is also added to check that video or audio is correctly
attached in the participants information.

Change-Id: I074cc4eb41b21418a6d751e3395db93e8a4da7f5
GitLab: #803