* Bug in `linkerd uninstall` when attempting to delete PSP
We were using a wrong apiVersion for PSP in `linkerd uninstall`'s
output, which avoids removing that resource:
```
$ linkerd uninstall | kubectl delete -f -
clusterrole.rbac.authorization.k8s.io "linkerd-linkerd-controller"
deleted
clusterrole.rbac.authorization.k8s.io "linkerd-linkerd-destination"
deleted
...
mutatingwebhookconfiguration.admissionregistration.k8s.io
"linkerd-proxy-injector-webhook-config" deleted
validatingwebhookconfiguration.admissionregistration.k8s.io
"linkerd-sp-validator-webhook-config" deleted
namespace "linkerd" deleted
error: unable to recognize "uninstall.yml": no matches for kind
"PodSecurityPolicy" in version "extensions/v1beta1"
$ kubectl get psp -oname
podsecuritypolicy.policy/linkerd-linkerd-control-plane
```
I've also replaced the uninstall integration test with a new separate
suite that performs the installation, waits for it to be ready,
uninstalls, and then confirms `linkerd check --pre` returns as expected.