Fix missing Mastodon/PosgreSQL backups

The backup cookbook was incomplete, and also there was no database
configured to be backed up.

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

@@ -17,7 +17,11 @@ KosmosBackup.new(:default, 'default backup') do
 <%- end -%>
 
 <%- if node["backup"]["postgresql"] -%>
-  database PostgreSQL
+<%- node["backup"]["postgresql"]["databases"].each do |db_name| -%>
+  database PostgreSQL, :"<%= db_name.to_sym %>" do |db|
+    db.name = "<%= db_name %>"
+  end
+<%- end -%>
 <%- end -%>
 
 <%- if node["mongodb"] -%>