func_config: Add ability to retrieve specific occurrence of a variable

I guess nobody uses templates with AST_CONFIG because today if you have a
context that inherits from a template and you call AST_CONFIG on the context,
you'll get the value from the template even if you've overridden it in the
context.  This is because AST_CONFIG only gets the first occurrence which is
always from the template.

This patch adds an optional 'index' parameter to AST_CONFIG which lets you
specify the exact occurrence to retrieve, or '-1' to retrieve the last.
The default behavior is the current behavior.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4313/
........

Merged revisions 430315 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430316 65c4cc65-6c06-0410-ace0-fbb531ad65f3