Allow specifying a channel to dial an extension and context in an ARI dial operation.

(issue ASTERISK-22625)
Reported by: Scott Griepentrog
........

Merged revisions 400254 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400255 65c4cc65-6c06-0410-ace0-fbb531ad65f3