[CAPI install] envtest.kubeconfig is not deleted when destroying cluster

Issue

• After destroying cluster, there are still some files leftover in <install-dir>/.clusterapi_output

  $ ls -ltra
  total 1516
  drwxr-xr-x. 1 fedora fedora     596 Jun 17 03:46 ..
  drwxr-x---. 1 fedora fedora      88 Jun 17 06:09 .clusterapi_output
  -rw-r--r--. 1 fedora fedora 1552382 Jun 17 06:09 .openshift_install.log
  drwxr-xr-x. 1 fedora fedora      80 Jun 17 06:09 . 
  
  $ ls -ltr .clusterapi_output/
  total 40
  -rw-r--r--. 1 fedora fedora  2335 Jun 17 05:58 envtest.kubeconfig
  -rw-r--r--. 1 fedora fedora 20542 Jun 17 06:03 kube-apiserver.log
  -rw-r--r--. 1 fedora fedora 10656 Jun 17 06:03 etcd.log
  

• Then continue installing new cluster within same install dir, installer exited with error as below:

  $ ./openshift-install create cluster --dir ipi-aws
  INFO Credentials loaded from the "default" profile in file "/home/fedora/.aws/credentials" 
  INFO Consuming Install Config from target directory 
  FATAL failed to fetch Cluster: failed to load asset "Cluster": local infrastructure provisioning artifacts already exist. There may already be a running cluster