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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@380193 f38db490-d61c-443f-a65b-d21fe96a405b