apps/confbridge: Added hear_own_join_sound option to control who hears sound_join

Added the hear_own_join_sound option to the confbridge user profile to
control who hears the sound_join audio file. When set to 'yes' the user
entering the conference and the participants already in the conference
will hear the sound_join audio file. When set to 'no' the user entering
the conference will not hear the sound_join audio file, but the
participants already in the conference will hear the sound_join audio
file.

ASTERISK-29931
Added by Michael Cargile

Change-Id: I856bd66dc0dfa057323860a6418c1371d249abd2