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kubeadm: Drop arch suffixes
**What this PR does / why we need it**:
This change removes arch suffixes from control plane images (etcd, kube-apiserver, kube-scheduler, kube-proxy, etc.). These are not needed, as almost all control plane images have a fat manifest now. It also adds a missing unit test for `GetGenericImage`
We have arch suffixes only for kube-dns images now.
Sample output of `kubeadm config images pull` with this change:
```
$ ./_output/bin/kubeadm config images pull
[config/images] Pulled k8s.gcr.io/kube-apiserver:v1.11.1
[config/images] Pulled k8s.gcr.io/kube-controller-manager:v1.11.1
[config/images] Pulled k8s.gcr.io/kube-scheduler:v1.11.1
[config/images] Pulled k8s.gcr.io/kube-proxy:v1.11.1
[config/images] Pulled k8s.gcr.io/pause:3.1
[config/images] Pulled k8s.gcr.io/etcd:3.2.18
[config/images] Pulled k8s.gcr.io/coredns:1.1.3
$ ./_output/bin/kubeadm config images pull --feature-gates=CoreDNS=false
[config/images] Pulled k8s.gcr.io/kube-apiserver:v1.11.1
[config/images] Pulled k8s.gcr.io/kube-controller-manager:v1.11.1
[config/images] Pulled k8s.gcr.io/kube-scheduler:v1.11.1
[config/images] Pulled k8s.gcr.io/kube-proxy:v1.11.1
[config/images] Pulled k8s.gcr.io/pause:3.1
[config/images] Pulled k8s.gcr.io/etcd:3.2.18
[config/images] Pulled k8s.gcr.io/k8s-dns-kube-dns-amd64:1.14.10
[config/images] Pulled k8s.gcr.io/k8s-dns-sidecar-amd64:1.14.10
[config/images] Pulled k8s.gcr.io/k8s-dns-dnsmasq-nanny-amd64:1.14.10
```
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Refs kubernetes/kubeadm#1030kubernetes/kubeadm#51
**Special notes for your reviewer**:
/cc @kubernetes/sig-cluster-lifecycle-pr-reviews
/area kubeadm
/kind feature
/assign @luxas
/assign @timothysc
/assign @dims
/assign @neolit123
**Release note**:
```release-note
kubeadm: Control plane images (etcd, kube-apiserver, kube-proxy, etc.) don't use arch suffixes. Arch suffixes are kept for kube-dns only.
```
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