Add the ability to dynamically specify weights for responses to DUNDi queries.

This can be done using a global variable or a dialplan function.  Using the
SHELL() function will allow you to use an external script to determine what the
weight in the response should be.  This can be very useful in load balancing
applications.
(inspired by discussions with blitzrage and jsmith in #asterisk-bugs)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@58304 65c4cc65-6c06-0410-ace0-fbb531ad65f3