Add support for 'setvar=' for MGCP device lines, like other channel drivers provide.

(closes issue #14818)
 Reported by: alea-soluciones
 Patches: 
       chan_mgcp-setvars-svn-trunk-r219899.patch uploaded by alea (license 514)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@219952 65c4cc65-6c06-0410-ace0-fbb531ad65f3