app_voicemail: use a consistent generator string

When updating voicemail.conf when a user changes
their pin, change the generator string to be the
same as the module name when reading so that the
same config_hook will be called.

Review: https://reviewboard.asterisk.org/r/3837/
........

Merged revisions 419284 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/11.6@419308 65c4cc65-6c06-0410-ace0-fbb531ad65f3