Add secret ability to specify zone

entrypoint/src/cmd/entrypoint/garage_util.go

@@ -265,9 +265,14 @@ func garageApplyLayout(
 
 			id := bootstrapGarageHostForAlloc(thisHost, alloc).ID
 
+			zone := hostName
+			if alloc.Zone != "" {
+				zone = alloc.Zone
+			}
+
 			clusterLayout[id] = peerLayout{
 				Capacity: alloc.Capacity / 100,
-				Zone:     hostName,
+				Zone:     zone,
 				Tags:     []string{},
 			}
 		}

entrypoint/src/daemon/config.go

@@ -36,6 +36,10 @@ type ConfigStorageAllocation struct {
 	S3APIPort int    `yaml:"s3_api_port"`
 	RPCPort   int    `yaml:"rpc_port"`
 	AdminPort int    `yaml:"admin_port"`
+
+	// Zone is a secret option which makes it easier to test garage bugs, but
+	// which we don't want users to otherwise know about.
+	Zone string `yaml:"zone"`
 }
 
 // Config describes the structure of the daemon config file.