sre.hosts.decommission: use mgmt IP if no DNS
- As the mgmt DNS record will be removed when running this cookbook, fallback to using the mgmt IP if the DNS fails so that the cookbook is idempotent and can be re-run on already decommissioned hosts.
Bug: T320721
Change-Id: Ie3a22548aec4c3917c8baf0f0bbf2165b6bfedf3