Add SessionProvisioner and InstanceProvisionerFactory

Remove AllowedImages and IsDockerHost as it is not really being used for
anything useful

handlers/get_instance_images.go

@@ -3,9 +3,15 @@ package handlers
 import (
 	"encoding/json"
 	"net/http"
+
+	"github.com/play-with-docker/play-with-docker/config"
 )
 
 func GetInstanceImages(rw http.ResponseWriter, req *http.Request) {
-	instanceImages := core.InstanceAllowedImages()
+	instanceImages := []string{
+		config.GetDindImageName(),
+		"franela/dind:overlay2-dev",
+		"franela/ucp:2.4.1",
+	}
 	json.NewEncoder(rw).Encode(instanceImages)
 }