`elbv2.AddTags` doesn't seem to support assigning the same set of
tags to multiple resources at once leading to the following error:
  Error adding tags after modifying load balancer targets:
  "ValidationError: Only one resource can be tagged at a time"

This can happen when using AWS NLB with multiple listeners pointing
to different node ports.

When k8s creates a NLB it creates a target group per listener along
with installing security group ingress rules allowing the traffic to
reach the k8s nodes.

Unfortunately if those target groups are not tagged, k8s will not
manage them, thinking it is not the owner.

This small changes assigns tags one resource at a time instead of
batching them as before.
Signed-off-by: Brice Figureau <brice@daysofwonder.com>