Fix swarm_labels

[dzimine]

Problem: if swarm_labels not defined at all, it fails on the last step with:

fatal: [node2]: FAILED! => {"failed": true, "msg": "'swarm_labels' is undefined"}

Using when swarm_labels is defined doesn't work with with_items. Let's use defaults instead.

[atosatto]

Thank you @dzimine!

[SergK]

@dzimine thank you for PR, yep it's good to have backward compatibility, afaik this was fixed in ansible/ansible#14383
@atosatto Probably ansible version should be reflected in Requirements section