[cern] K8S: Allows to specify admission control plugins to enable

Fixes #OS-3736.

Cherry-pick: https://review.openstack.org/#/c/405374/

If nothing is specified a set of recommended default plugins is used, which includes the ServiceAccount one.

Change-Id: I1383aae09ba68f8e83b07e3eaae40ab071f7be94 Closes-Bug: #1646489

Conflicts: doc/source/userguide.rst magnum/drivers/common/k8s_template_def.py magnum/drivers/common/templates/kubernetes/fragments/configure-kubernetes-master.sh magnum/drivers/k8s_coreos_v1/templates/kubecluster.yaml magnum/drivers/k8s_fedora_atomic_v1/templates/kubecluster.yaml magnum/drivers/k8s_fedora_atomic_v1/templates/kubemaster.yaml magnum/drivers/k8s_fedora_ironic_v1/templates/kubecluster.yaml magnum/drivers/k8s_fedora_ironic_v1/templates/kubemaster.yaml magnum/tests/functional/k8s/test_k8s_python_client.py magnum/tests/unit/conductor/handlers/test_k8s_cluster_conductor.py magnum/tests/unit/drivers/test_template_definition.py

Closes #3736