conference: prevent a call to be added twice

Fix Manager::addParticipant() to always accept (return true)
to add a call already in given conference (no-op).
Without this patch, doing so causes a daemon crash.

Change-Id: I7b03a1349e7cac148ba3991b8e0bc2a9f1521c85
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>