func_config: Change 'Not Found' message from ERROR to DEBUG

When you call the CONFIG dialplan function with the name of a variable that
doesn't exist in the target context you get an ERROR.  This does nothing but
clutter up the logs with messages that may be perfectly acceptable.  Just
because a variable wasn't in the context doesn't mean it's an error.  Maybei
t's optional or just needs to be defaulted or ignored.

This patch changes the log level from ERROR to DEBUG.  If a dialplan developer
wants to debug their dialplan they still canby setting the console debug level 
as needed.

Tested by: George Joseph
Review: https://reviewboard.asterisk.org/r/3919/
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Merged revisions 421327 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 421328 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@421329 f38db490-d61c-443f-a65b-d21fe96a405b