Change ARI user config to use a type field

Change ARI user config to use a type field

When I initially wrote the configuration support for ARI users, I
determined the section type by a category prefix (i.e., [user-admin]).

This is neither idiomatic Asterisk configuration, nor is it really
that user friendly. This patch replaces the category prefix with a
type field in the section, which is much cleaner.

Review: https://reviewboard.asterisk.org/r/2664/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394076 65c4cc65-6c06-0410-ace0-fbb531ad65f3