Do not pass the password on the command line anymore to fix a warning

Since email notifications work now we do not want warnings. Write an
option file with the credentials for mysqldump
(https://dev.mysql.com/doc/refman/5.7/en/option-files.html)

site-cookbooks/backup/templates/default/config.rb.erb

@@ -40,7 +40,6 @@ end
 Database::MySQL.defaults do |db|
   db.host               = "<%= node["backup"]["mysql"]["host"] %>"
   db.username           = "<%= node["backup"]["mysql"]["username"] %>"
-  db.password           = "<%= node["backup"]["mysql"]["password"] %>"
   db.additional_options = ['--quick', '--single-transaction']
 end
 <%- end -%>