change coredns image name to `coredns/coredns` and prefix `v` to tag (#7570)

follow new naming conventions for gcr's coredns image.
starting from 1.21 kubeadm assumes it to be `coredns/coredns`:
this causes the kubeadm deployment being unable to pull image, beacuse `v`
was also added in image tag, until the role `kubernetes-apps` ovverides
it with the old name, which is only compatible with <=1.7.

Backward comptability with kubeadm <=1.20 is mantained checking
kubernetes version and falling back to old names (`coredns:1.xx`) when
the version is less than 1.21

roles/download/defaults/main.yml

@@ -447,9 +447,11 @@ haproxy_image_tag: 2.3
 
 # Coredns version should be supported by corefile-migration (or at least work with)
 # bundle with kubeadm; if not 'basic' upgrade can sometimes fail
-coredns_version: "1.7.0"
+coredns_image_is_namespaced: "{{ (kube_major_version | regex_replace('^v', '') | float) >= 1.21 }}"
-coredns_image_repo: "{{ kube_image_repo }}/coredns"
+
-coredns_image_tag: "{{ coredns_version }}"
+coredns_version: "v1.7.0"
+coredns_image_repo: "{{ kube_image_repo }}{{'/coredns/coredns' if (coredns_image_is_namespaced | bool) else '/coredns' }}"
+coredns_image_tag: "{{ coredns_version if (coredns_image_is_namespaced | bool) else (coredns_version | regex_replace('^v', '')) }}"
 
 nodelocaldns_version: "1.17.1"
 nodelocaldns_image_repo: "{{ kube_image_repo }}/dns/k8s-dns-node-cache"

roles/download/templates/kubeadm-images.yaml.j2

@@ -21,5 +21,5 @@ etcd:
 {% endif %}
 dns:
   type: CoreDNS
-  imageRepository: {{ coredns_image_repo | regex_replace('/coredns$','') }}
+  imageRepository: {{ coredns_image_repo | regex_replace('/coredns.*$','') }}
   imageTag: {{ coredns_image_tag }}

roles/kubernetes/control-plane/templates/kubeadm-config.v1beta2.yaml.j2

@@ -84,7 +84,7 @@ etcd:
 {% endif %}
 dns:
   type: CoreDNS
-  imageRepository: {{ coredns_image_repo | regex_replace('/coredns$','') }}
+  imageRepository: {{ coredns_image_repo | regex_replace('/coredns.*$','') }}
   imageTag: {{ coredns_image_tag }}
 networking:
   dnsDomain: {{ dns_domain }}