Permit storage of voicemail secrets in a separate file, located within the spool directory.

(closes issue #14276)
 Reported by: klaus3000
 Patches: 
       app_voicemail.c-svn-trunk-r214898.txt uploaded by klaus3000 (license 65)
 Tested by: jamesgolovich


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@225406 65c4cc65-6c06-0410-ace0-fbb531ad65f3