Added an option to disallow music on hold

Added an option "discard_remote_hold_retrieval" (default "no") that if set does
not trigger the music on hold event.  This essentially stops telling the peer
to start music on hold.

(issue ABE-2899)
Reported by: Denis Alberto Martinez
Review: https://reviewboard.asterisk.org/r/2336/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@382787 65c4cc65-6c06-0410-ace0-fbb531ad65f3