Ansible var should be quoted (#3393)

to fix the follow problem in case quote is not used:

PLAY [k8s-cluster:etcd:calico-rr] **********************************************
ERROR! Syntax Error while loading YAML.
  expected <block end>, but found '<scalar>'

The error appears to have been in '/tmp/vagrant-ansible/inventory/group_vars/k8s-cluster.yml': line 59, column 39, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

kube_oidc_ca_file: {{ kube_cert_dir }}/openid-ca.pem
                                      ^ here
We could be wrong, but this one looks like it might be an issue with
missing quotes.  Always quote template expression brackets when they
start a value. For instance:

    with_items:
      - {{ foo }}

Should be written as:

    with_items:
      - "{{ foo }}"

Ansible failed to complete successfully. Any error output should be
visible above. Please fix these errors and try again.