Value of Cache.Url may be ignored even when set explicitly

[jhiemstrawisc]

It looks like the parameter Cache.Url isn't respected everywhere it should be, at least in the case of self-tests.

While setting Cache.Port does cause the Cache.Url to update its port, the inability to set Cache.Url directly means a containerized cache can't separate internal container ports from external world-accessible ports.

Cache.Port should define the port the cache's XRootD process binds to wherever it happens to be running
Cache.Url should define the external hostname+port services can reach the cache at.

This breaks the way we deploy some caches in Kubernetes, where we do lots of external-to-internal port mapping.