config: inform config hook of change when writing file

When updated configuration is written back to the conf
file - for example when a user changes their voicemail
pin, make sure that any config hook that wants to know
of changes is informed.

Review: https://reviewboard.asterisk.org/r/3708/
........

Merged revisions 418366 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@418369 f38db490-d61c-443f-a65b-d21fe96a405b