aws: Graceful handling of EC2 detach errors

Sometimes, we observe the following error during a rolling update:

error detaching instance "i-XXXX", node "ip-10-X-X-X.ec2.internal": error detaching instance "i-XXXX": ValidationError: The instance i-XXXX is not part of Auto Scaling group XXXXX

The sequence of events that lead to this problem is the following:

- A new ASG object is being built from the launch template
- Existing instances are being added to it
- An existing instance is being ignored because it's already terminating
W0205 08:01:32.593377     191 aws_cloud.go:791] ignoring instance as it is terminating: i-XXXX in autoscaling group: XXXX
- Due to maxSurge, the terminating instance is trying to be detached
  from the autoscaling group and fails.

As such, in case of EC@ ASG deatch failures we can simply try to detach
the next node instead of aborting the whole update operation.

pkg/instancegroups/instancegroups.go

@@ -134,11 +134,15 @@ func (c *RollingUpdateCluster) rollingUpdateInstanceGroup(group *cloudinstances.
 	update = prioritizeUpdate(update)
 
 	if maxSurge > 0 && !c.CloudOnly {
+		skippedNodes := 0
 		for numSurge := 1; numSurge <= maxSurge; numSurge++ {
-			u := update[len(update)-numSurge]
+			u := update[len(update)-numSurge+skippedNodes]
 			if u.Status != cloudinstances.CloudInstanceStatusDetached {
 				if err := c.detachInstance(u); err != nil {
-					return err
+					// If detaching a node fails, we simply proceed to the next one instead of
+					// bubbling up the error.
+					skippedNodes++
+					numSurge--
 				}
 
 				// If noneReady, wait until after one node is detached and its replacement validates