DNS resolution validation

When creating and updating services configured with domain mappings, MedStack Control now automatically checks that domains resolve to the IP address of the cluster Manager node. DNS records that fail to resolve to the service trigger an error preventing the service from being updated.

This change introduces a stability improvement that prevents Let's Encrypt from timing out due to misconfiguring DNS records and Docker services, as well as a security improvement to mitigate certificates holding on to decommissioned domains.