Multiple playgrounds support (#215)

* Add Playground struct and basic support for creating it and retrieving
it

* Add missing functions in pwd mock

* Get playground from request domain and validate it exists. If valid set
it on the newly created session.

* Move playground specific configurations to the playground struct and use
it everytime we need that conf.

* Don't allow to specify a duration bigger that the allowed in the
playground

handlers/get_instance_images.go

@@ -2,15 +2,16 @@ package handlers
 
 import (
 	"encoding/json"
+	"log"
 	"net/http"
-
-	"github.com/play-with-docker/play-with-docker/config"
 )
 
 func GetInstanceImages(rw http.ResponseWriter, req *http.Request) {
-	instanceImages := []string{
-		config.GetDindImageName(),
-		"franela/dind:dev",
+	playground := core.PlaygroundFindByDomain(req.Host)
+	if playground == nil {
+		log.Printf("Playground for domain %s was not found!", req.Host)
+		rw.WriteHeader(http.StatusBadRequest)
+		return
 	}
-	json.NewEncoder(rw).Encode(instanceImages)
+	json.NewEncoder(rw).Encode(playground.AvailableDinDInstanceImages)
 }