Factor out API defaulting from validation logic

Currently, the validation logic validates fields in an object and supply default
values wherever applies. This change factors out defaulting to a set of
defaulting callback functions for decoding (see #1502 for more discussion).

 * This change is based on pull request 2587.

 * Most defaulting has been migrated to defaults.go where the defaulting
   functions are added.

 * validation_test.go and converter_test.go have been adapted to not testing the
   default values.

 * Fixed all tests with that create invalid objects with the absence of
   defaulting logic.

pkg/master/publish.go

@@ -90,8 +90,10 @@ func (m *Master) createMasterServiceIfNeeded(serviceName string, serviceIP net.I
 		Spec: api.ServiceSpec{
 			Port: servicePort,
 			// maintained by this code, not by the pod selector
-			Selector: nil,
-			PortalIP: serviceIP.String(),
+			Selector:        nil,
+			PortalIP:        serviceIP.String(),
+			Protocol:        api.ProtocolTCP,
+			SessionAffinity: api.AffinityTypeNone,
 		},
 	}
 	// Kids, don't do this at home: this is a hack. There's no good way to call the business