This allows manual setting of the cluster's domain name. I use this to
make the local cluster admin aware that this has been done.
(and then manually update the external dns)
```
% uc dns set ctrl.u.science.ru.nl
Set cluster domain: ctrl.u.science.ru.nl
Deploy the Caddy reverse proxy service ('uc caddy deploy') to enable internet access to your services via the reserved or your custom domain.
% uc dns show
ctrl.u.science.ru.nl
% uc dns set ctrl.u.science.ru.nl
Error: domain already reserved
% uc dns release
Released cluster domain: ctrl.u.science.ru.nl
```
See: #37
Signed-off-by: Miek Gieben <miek@miek.nl>
Pasha Sviderski