mantle/platform/gcloud: fix confidential compute check

In db803c3 we add support for ConfidentialType but we made it so
that we always execute this code where we don't want to do that if
the user didn't specify any ConfidentialType for GCP.

The way the code is now we can't run any GCP tests on non confidential
instances.

```
failed to create instance "kola-dd144aac7413f66c85e9":
    Does not support confidential type , should be: sev, sev_snp
```

Author: dustymabe

mantle/platform/api/gcloud/compute.go

@@ -147,18 +147,20 @@ func (a *API) mkinstance(userdata, name string, keys []*agent.Key, opts platform
 		})
 	}
 	// create confidential instance
-	ConfidentialType := strings.ToUpper(a.options.ConfidentialType)
-	ConfidentialType = strings.Replace(ConfidentialType, "-", "_", -1)
-	if ConfidentialType == "SEV" || ConfidentialType == "SEV_SNP" {
-		fmt.Printf("Using confidential type for confidential computing %s\n", ConfidentialType)
-		instance.ConfidentialInstanceConfig = &compute.ConfidentialInstanceConfig{
-			ConfidentialInstanceType: ConfidentialType,
-		}
-		instance.Scheduling = &compute.Scheduling{
-			OnHostMaintenance: "TERMINATE",
+	if a.options.ConfidentialType != "" {
+		ConfidentialType := strings.ToUpper(a.options.ConfidentialType)
+		ConfidentialType = strings.Replace(ConfidentialType, "-", "_", -1)
+		if ConfidentialType == "SEV" || ConfidentialType == "SEV_SNP" {
+			fmt.Printf("Using confidential type for confidential computing %s\n", ConfidentialType)
+			instance.ConfidentialInstanceConfig = &compute.ConfidentialInstanceConfig{
+				ConfidentialInstanceType: ConfidentialType,
+			}
+			instance.Scheduling = &compute.Scheduling{
+				OnHostMaintenance: "TERMINATE",
+			}
+		} else {
+			return nil, fmt.Errorf("Does not support confidential type %s, should be: sev, sev_snp\n", a.options.ConfidentialType)
 		}
-	} else {
-		return nil, fmt.Errorf("Does not support confidential type %s, should be: sev, sev_snp\n", a.options.ConfidentialType)
 	}
 	// attach aditional disk
 	for _, spec := range opts.AdditionalDisks {