ui: implement tag ordering as configured in datacenter config

ui: implement tag ordering as configured in datacenter config

datacenter.cfg returns an 'ordering' option. parse that and
use it to order the tags when viewing. default is alphabetical.

With alphabetical ordering, drag & drop when editing is disabled and the
tags will be inserted at the right place. When saving, the sorted
order will be written into the config

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>