cloudapi: make Bucket optional in GCPUploadOptions

GCP Bucket to use can be now configured in the worker configuration.
Make the `Bucket` optional in the Cloud API when uploading image to GCP.

Adjust the Cloud API test case to configure GCP Bucket on the worker and
not provide it in the API request.

internal/cloudapi/v2/handler.go

@@ -448,10 +448,14 @@ func (h *apiHandlers) PostCompose(ctx echo.Context) error {
 				}
 
 				imageName := fmt.Sprintf("composer-api-%s", uuid.New().String())
+				var bucket string
+				if gcpUploadOptions.Bucket != nil {
+					bucket = *gcpUploadOptions.Bucket
+				}
 				t := target.NewGCPTarget(&target.GCPTargetOptions{
 					Region: gcpUploadOptions.Region,
 					Os:     imageType.Arch().Distro().Name(), // not exposed in cloudapi
-					Bucket: gcpUploadOptions.Bucket,
+					Bucket: bucket,
 					// the uploaded object must have a valid extension
 					Object:            fmt.Sprintf("%s.tar.gz", imageName),
 					ShareWithAccounts: share,