Disable readyReplicas validation for Deployments

Because there is no field in 1.5, when we update to 1.6 and the
controller tries to update the Deployment, it will be denied by
validation because the pre-existing availableReplicas field is greater
than readyReplicas (normally readyReplicas should always be greater or
equal).

pkg/apis/extensions/validation/validation.go

@@ -352,7 +352,9 @@ func ValidateDeploymentStatus(status *extensions.DeploymentStatus, fldPath *fiel
 	if status.AvailableReplicas > status.Replicas {
 		allErrs = append(allErrs, field.Invalid(fldPath.Child("availableReplicas"), status.AvailableReplicas, msg))
 	}
-	if status.AvailableReplicas > status.ReadyReplicas {
+	// TODO: ReadyReplicas is introduced in 1.6 and this check breaks the Deployment controller when pre-1.6 clusters get upgraded.
+	// 		 Remove the comparison to zero once we stop supporting upgrades from 1.5.
+	if status.ReadyReplicas > 0 && status.AvailableReplicas > status.ReadyReplicas {
 		allErrs = append(allErrs, field.Invalid(fldPath.Child("availableReplicas"), status.AvailableReplicas, "cannot be greater than readyReplicas"))
 	}
 	return allErrs

pkg/apis/extensions/validation/validation_test.go

@@ -1290,6 +1290,15 @@ func TestValidateDeploymentStatus(t *testing.T) {
 			observedGeneration: 1,
 			expectedErr:        true,
 		},
+		// TODO: Remove the following test case once we stop supporting upgrades from 1.5.
+		{
+			name:               "don't validate readyReplicas when it's zero",
+			replicas:           3,
+			readyReplicas:      0,
+			availableReplicas:  3,
+			observedGeneration: 1,
+			expectedErr:        false,
+		},
 	}
 
 	for _, test := range tests {