config hooks: correct ref leaks

This small patch fixes a ref leak when
adding a config hook and cleans up the
container on shutdown.

Review: https://reviewboard.asterisk.org/r/4407



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@431582 65c4cc65-6c06-0410-ace0-fbb531ad65f3