Disable admission controllers for webhook server (#468)

Since [1.10 release](https://github.com/kubernetes/apiserver/blob/release-1.10/pkg/server/options/recommended.go#L43) admission options are enabled by default . This was not the case in 1.9 release. Admission plugins seem unnecessary for a webhook server. So, I am disabling it.

If this is left enabled, then RBAC permissions need to be updated accordingly.
```
- apiGroups:
  - admissionregistration.k8s.io
  resources:
    - mutatingwebhookconfigurations
    - validatingwebhookconfigurations
  verbs: ["get","list"]
```

server/start.go

@@ -33,6 +33,7 @@ func NewStashOptions(out, errOut io.Writer) *StashOptions {
 		StdErr:             errOut,
 	}
 	o.RecommendedOptions.Etcd = nil
+	o.RecommendedOptions.Admission = nil
 
 	return o
 }