feat: add uc dns set

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>

api/pb/cluster.proto

@@ -14,6 +14,7 @@ service Cluster {
   rpc RemoveMachine(RemoveMachineRequest) returns (google.protobuf.Empty);
 
   rpc ReserveDomain(ReserveDomainRequest) returns (Domain);
+  rpc SetDomain(SetDomainRequest) returns (Domain);
   rpc GetDomain(google.protobuf.Empty) returns (Domain);
   rpc ReleaseDomain(google.protobuf.Empty) returns (Domain);
   rpc CreateDomainRecords(CreateDomainRecordsRequest) returns (CreateDomainRecordsResponse);
@@ -62,6 +63,10 @@ message ReserveDomainRequest {
   string endpoint = 1;
 }
 
+message SetDomainRequest {
+  string name = 1;
+}
+
 message CreateDomainRecordsRequest {
   repeated DNSRecord records = 1;
 }