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

pwd/playground.go

@@ -0,0 +1,36 @@
+package pwd
+
+import (
+	"log"
+
+	"github.com/play-with-docker/play-with-docker/pwd/types"
+	"github.com/twinj/uuid"
+)
+
+func (p *pwd) PlaygroundNew(playground types.Playground) (*types.Playground, error) {
+	playground.Id = uuid.NewV5(uuid.NameSpaceURL, uuid.Name(playground.Domain)).String()
+	if err := p.storage.PlaygroundPut(&playground); err != nil {
+		log.Printf("Error saving playground %s. Got: %v\n", playground.Id, err)
+		return nil, err
+	}
+
+	return &playground, nil
+}
+
+func (p *pwd) PlaygroundGet(id string) *types.Playground {
+	if playground, err := p.storage.PlaygroundGet(id); err != nil {
+		log.Printf("Error retrieving playground %s. Got: %v\n", id, err)
+		return nil
+	} else {
+		return playground
+	}
+}
+
+func (p *pwd) PlaygroundFindByDomain(domain string) *types.Playground {
+	id := uuid.NewV5(uuid.NameSpaceURL, uuid.Name(domain)).String()
+	return p.PlaygroundGet(id)
+}
+
+func (p *pwd) PlaygroundList() ([]*types.Playground, error) {
+	return p.storage.PlaygroundGetAll()
+}