e2e.go / kops: Add ssh-key option to override ssh key

By default, Jenkins stuffs the ssh key in
/workspace/.aws/kube_aws_rsa. Allow this to be overridden easily on
the command line.

hack/verify-flags/known-flags.txt

@@ -276,6 +276,7 @@ keep-gogoproto
 km-path
 kops-cluster
 kops-nodes
+kops-ssh-key
 kops-state
 kops-zones
 kube-api-burst