Split out the Package type into a first class Kubernetes resource (#295)

Split out the Package type into a first class Kubernetes resource. Before this change, packages were implicitly tied to functions.

This wasn't ideal because:
 * Functions will need to share packages
 * A package storage system may be more generally useful than just
   functions (for example, for storing static assets)

This change does the following:
* Updates the fission and tpr types to add a new Package and PackageSpec.  It also creates a PackageRef type, and a FunctionPackageRef type. The PackageRef simply references a package, but the FunctionPackageRef includes the name of a function within the package. This allows us to share packages between different functions.
* Updates fetcher and other components for first-class packages
* Allows customization of fetcher image pull policy in the helm charts

tpr/tpr_test.go

@@ -46,9 +46,13 @@ func functionTests(tprClient *rest.RESTClient) {
 			Name: "hello",
 		},
 		Spec: fission.FunctionSpec{
-			Source: fission.Package{},
-			Deployment: fission.Package{
-				Literal: []byte("hi"),
+			Source: fission.FunctionPackageRef{},
+			Deployment: fission.FunctionPackageRef{
+				PackageRef: fission.PackageRef{
+					Name:      "foo",
+					Namespace: "bar",
+				},
+				FunctionName: "hello",
 			},
 			EnvironmentName: "xxx",
 		},
@@ -70,7 +74,7 @@ func functionTests(tprClient *rest.RESTClient) {
 	// read
 	f, err = fi.Get(function.Metadata.Name)
 	panicIf(err)
-	if len(f.Spec.Deployment.Literal) != len(function.Spec.Deployment.Literal) {
+	if f.Spec.Deployment.FunctionName != function.Spec.Deployment.FunctionName {
 		log.Panicf("Bad result from Get: %v", f)
 	}