Fix ecs.Cluster external name configuration

[ulucinar]

Description of your changes

Fixes ecs.Cluster external name configuration as reported in #168.

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Tested provisioning and deleting a resource using the manifest:

apiVersion: ecs.aws.jet.crossplane.io/v1alpha2
kind: Cluster
metadata:
  name: alper-sample-cluster
spec:
  forProvider:
    region: us-west-2

❯ k get clusters.ecs -w
NAME                   READY   SYNCED   EXTERNAL-NAME          AGE
alper-sample-cluster                    alper-sample-cluster   3s
alper-sample-cluster                    alper-sample-cluster   11s
alper-sample-cluster                    alper-sample-cluster   11s
alper-sample-cluster                    alper-sample-cluster   11s
alper-sample-cluster   False   True     alper-sample-cluster   11s
alper-sample-cluster   False   True     alper-sample-cluster   11s
alper-sample-cluster   False   True     alper-sample-cluster   30s
alper-sample-cluster   False   True     alper-sample-cluster   35s
alper-sample-cluster   True    True     alper-sample-cluster   39s
alper-sample-cluster   True    True     alper-sample-cluster   2m30s

Also provisioned, then orphaned and deleted a resource, and then could successfully re-import it.