app_confbridge: Set the language used for announcements to the conference.

ConfBridge now has the ability to set the language of announcements to the
conference.  The language can be set on a bridge profile in
confbridge.conf or by the dialplan function
CONFBRIDGE(bridge,language)=en.

(closes issue ASTERISK-19983)
Reported by: Jonathan White
Patches:
      M19983_rev2.diff (license #5138) patch uploaded by junky (modified)
Tested by: rmudgett

Change-Id: Ibb77668ecfa626f66aa0eae6d555c516a1d5cd32