Delete fails when ingress does not exist in a cluster

[nikhiljindal]

Steps to repro:

• Create an MCI in some clusters
• Remove the MCI from all those clusters
• Run delete to delete the MCI

Expected:

• Delete deletes the remaining GCP resources associated with that MCI

Actual:

• Fails with ingress not found in that cluster.

I tried delete again with --force and got unknown flag --force.

Steps to fix:

• We should support --force in delete and allow user to delete whatever is left.

cc @G-Harmon @csbell

[nikhiljindal]

Sent https://github.com/GoogleCloudPlatform/k8s-multicluster-ingress/pull/168 to fix this