kubeadm-alpha: CoreDNS related changes

1. Remove `..kube-dns.md` in favor of `..coredns.md`
This is a side-effect of renaming the sub-command property.
2. Update info text about using the `CoreDNS` feature gate
3. Add note that CoreDNS is the default DNS in 1.11.

content/en/docs/reference/setup-tools/kubeadm/generated/kubeadm_alpha_phase_addon_coredns.md

@@ -0,0 +1,31 @@
+
+Installs the CoreDNS addon to a Kubernetes cluster
+
+### Synopsis
+
+Installs the CoreDNS addon components via the API server. Please note that although the DNS server is deployed, it will not be scheduled until CNI is installed. 
+
+Alpha Disclaimer: this command is currently alpha.
+
+```
+kubeadm alpha phase addon coredns [flags]
+```
+
+### Options
+
+```
+      --config string               Path to a kubeadm config file. WARNING: Usage of a configuration file is experimental
+      --feature-gates string        A set of key=value pairs that describe feature gates for various features.Options are:
+                                    Auditing=true|false (ALPHA - default=false)
+                                    CoreDNS=true|false (default=true)
+                                    DynamicKubeletConfig=true|false (ALPHA - default=false)
+                                    SelfHosting=true|false (ALPHA - default=false)
+                                    StoreCertsInSecrets=true|false (ALPHA - default=false)
+  -h, --help                        help for coredns
+      --image-repository string     Choose a container registry to pull control plane images from (default "k8s.gcr.io")
+      --kubeconfig string           The KubeConfig file to use when talking to the cluster (default "/etc/kubernetes/admin.conf")
+      --kubernetes-version string   Choose a specific Kubernetes version for the control plane (default "stable-1.10")
+      --service-cidr string         The range of IP address used for service VIPs (default "10.96.0.0/12")
+      --service-dns-domain string   Alternative domain for services (default "cluster.local")
+```
+

content/en/docs/reference/setup-tools/kubeadm/kubeadm-alpha.md

@@ -129,12 +129,14 @@ Alternatively, you can use [kubeadm config](kubeadm-config.md).
 You can install all the available addons with the `all` subcommand, or
 install them selectively.
 
-Please note that if kubeadm is invoked with `--feature-gates=CoreDNS=true`,  [CoreDNS](https://coredns.io/) is installed instead of `kube-dns`.
+{{< note >}}
+**Note:** If `kubeadm` is invoked with `--feature-gates=CoreDNS=false`, kube-dns is installed.
+{{< /note >}}
 
 {{< tabs name="tab-addon" >}}
 {{< tab name="all" include="generated/kubeadm_alpha_phase_addon_all.md" />}}
 {{< tab name="kube-proxy" include="generated/kubeadm_alpha_phase_addon_kube-proxy.md" />}}
-{{< tab name="kube-dns" include="generated/kubeadm_alpha_phase_addon_kube-dns.md" />}}
+{{< tab name="coredns" include="generated/kubeadm_alpha_phase_addon_coredns.md" />}}
 {{< /tabs >}}