mod_voicemail: Add 2 new profile settings, db-password-override and allow-empty-passw...

mod_voicemail: Add 2 new profile settings, db-password-override and allow-empty-password-auth.  By default, they have value of their previous behavior.  If db-password-override=true, the db password will only be used if present, if not present fallback to the xml config file vm-password.  If allow-empty-password-auth=false, it will disable login via a authentication method if there is no password set in the user account (This wont affect voicemail_authorize=true login).