Fix res_ari_asterisk load issue

The new res_ari_asterisk.so module presents several config options
from asterisk main. Unfortunately, they aren't exported, so the module
won't load on Linux.

This patch renames the variables, adding the ast_ prefix so they will
be exported.

Review: https://reviewboard.asterisk.org/r/2737


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396166 65c4cc65-6c06-0410-ace0-fbb531ad65f3