Use nodepool.cloud for zuul_site_mirror_fqdn
We actually want to be using the cloud name, not provider name when
building up our fqdn for mirrors.
# This file contains ansible variables that are used by our zuul-executors.
---
-zuul_site_mirror_fqdn: "mirror.{{ nodepool.region | lower }}.{{ nodepool.provider | lower }}.openstack.org"
+zuul_site_mirror_fqdn: "mirror.{{ nodepool.region | lower }}.{{ nodepool.cloud | lower }}.openstack.org"
