[os_swift] Verify md5sum of builder files task fails if --limit flag is used

[Ihor Kalnytskyi]

'Verify md5sum of builder files' task [1] fails if Ansible execution is limited to some set of hosts. This happens because the task [1] is trying to access 'builder_md5sum' fact of all Swift hosts, but it's not there if we execute a playbook for a limited set of hosts.

In other words, I can't run 'os-swift-install.yml' playbook and limit its execution to (let's say) one host out of five.

[1] https://github.com/openstack/openstack-ansible-os_swift/blob/15.0.0.0b1/tasks/swift_rings_post_distribution_check.yml#L22-L30