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/client.go

@@ -138,6 +138,13 @@ func configureClient(config *rest.Config) {
 				&api.ListOptions{},
 				&api.DeleteOptions{},
 			)
+			scheme.AddKnownTypes(
+				groupversion,
+				&Package{},
+				&PackageList{},
+				&api.ListOptions{},
+				&api.DeleteOptions{},
+			)
 			return nil
 		})
 	schemeBuilder.AddToScheme(api.Scheme)
@@ -193,6 +200,10 @@ func (fc *FissionClient) Timetriggers(ns string) TimetriggerInterface {
 func (fc *FissionClient) Messagequeuetriggers(ns string) MessagequeuetriggerInterface {
 	return MakeMessagequeuetriggerInterface(fc.tprClient, ns)
 }
+func (fc *FissionClient) Packages(ns string) PackageInterface {
+	return MakePackageInterface(fc.tprClient, ns)
+}
+
 func (fc *FissionClient) WaitForTPRs() {
 	waitForTPRs(fc.tprClient)
 }