Change ARI originate to also allow dialing an exten/context/priority.

The old way didn't make much sense, so some of the fields were repurposed.

(closes issue ASTERISK-21658)

Review: https://reviewboard.asterisk.org/r/2626/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@393144 f38db490-d61c-443f-a65b-d21fe96a405b