Support for both map-based and set-based selectors in extensions/v1beta1.Scale

Here are a list of changes along with an explanation of how they work:
1. Add a new string field called TargetSelector to the external version of
   extensions Scale type (extensions/v1beta1.Scale). This is a serialized
   version of either the map-based selector (in case of ReplicationControllers)
   or the unversioned.LabelSelector struct (in case of Deployments and
   ReplicaSets).
2. Change the selector field in the internal Scale type (extensions.Scale) to
   unversioned.LabelSelector.
3. Add conversion functions to convert from two external selector fields to a
   single internal selector field. The rules for conversion are as follows:
   i.   If the target resource that this scale targets supports LabelSelector
        (Deployments and ReplicaSets), then serialize the LabelSelector and
        store the string in the TargetSelector field in the external version
        and leave the map-based Selector field as nil.
   ii.  If the target resource only supports a map-based selector
        (ReplicationControllers), then still serialize that selector and
	store the serialized string in the TargetSelector field. Also,
	set the the Selector map field in the external Scale type.
   iii. When converting from external to internal version, parse the
        TargetSelector string into LabelSelector struct if the string isn't
	empty. If it is empty, then check if the Selector map is set and just
	assign that map to the MatchLabels component of the LabelSelector.
   iv.  When converting from internal to external version, serialize the
        LabelSelector and store it in the TargetSelector field. If only
	the MatchLabel component is set, then also copy that value to
	the Selector map field in the external version.
4. HPA now just converts the LabelSelector field to a Selector interface
   type to list the pods.
5. Scale Get and Update etcd methods for Deployments and ReplicaSets now
   return extensions.Scale instead of autoscaling.Scale.
6. Consequently, SubresourceGroupVersion override and is "autoscaling"
   enabled check is now removed from pkg/master/master.go
7. Other small changes to labels package, fuzzer and LabelSelector
   helpers to piece this all together.
8. Add unit tests to HPA targeting Deployments and ReplicaSets.
9. Add an e2e test to HPA targeting ReplicaSets.

pkg/api/unversioned/helpers_test.go

@@ -46,7 +46,7 @@ func TestLabelSelectorAsSelector(t *testing.T) {
 		{in: &LabelSelector{}, out: labels.Everything()},
 		{
 			in:  &LabelSelector{MatchLabels: matchLabels},
-			out: mustParse("foo in (bar)"),
+			out: mustParse("foo=bar"),
 		},
 		{
 			in:  &LabelSelector{MatchExpressions: matchExpressions},
@@ -54,7 +54,7 @@ func TestLabelSelectorAsSelector(t *testing.T) {
 		},
 		{
 			in:  &LabelSelector{MatchLabels: matchLabels, MatchExpressions: matchExpressions},
-			out: mustParse("foo in (bar),baz in (norf,qux)"),
+			out: mustParse("baz in (norf,qux),foo=bar"),
 		},
 		{
 			in: &LabelSelector{