Only make a PVC for a PipelineRun if we need to. 🗑️

We only need to make a PVC if a Pipeline contains tasks that declare Output
resources of the allowed types. This PR changges our detection to only create
a PVC under those conditions.

This was originally attempted in tektoncd#1007 and
then subsequently rolled back in tektoncd#1071. I
*think* this one gets the logic correct :)

Co-authored-by: Christie Wilson <bobcatfish@gmail.com>
Fixes: tektoncd#937

pkg/apis/pipeline/v1alpha1/resource_types.go

@@ -30,6 +30,13 @@ import (
 // additional metatdata should be provided for it.
 type PipelineResourceType string
 
+var (
+	AllowedOutputResources = map[PipelineResourceType]bool{
+		PipelineResourceTypeStorage: true,
+		PipelineResourceTypeGit:     true,
+	}
+)
+
 const (
 	// PipelineResourceTypeGit indicates that this source is a GitHub repo.
 	PipelineResourceTypeGit PipelineResourceType = "git"