Fix Some Configured Conference Bridge Sounds Not Being Set

Fix Some Configured Conference Bridge Sounds Not Being Set

The "sound_only_one" sound was not being set even though it was configured.  In
looking into this, I found that the "join" and "leave" prompts were not being
set either.

(closes issue ASTERISK-20898)
Reported by: Stephan
Tested by: Stephan
Patches:
    asterisk-20898-custom-sounds-ignored.diff uploaded by
                                                 Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2289/
........

Merged revisions 380193 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@380194 65c4cc65-6c06-0410-ace0-fbb531ad65f3