Allow greetings-only mailboxes for Voicemail.

(closes issue #15132)
 Reported by: floletarmo
 Patches: 
       voicemail_changes.patch uploaded by floletarmo (license 784)
       (with some additional changes by me)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@234820 65c4cc65-6c06-0410-ace0-fbb531ad65f3