22777: Refactor container HTTP proxy routing.

22777: Refactor container HTTP proxy routing.

Now the router identifies ContainerHTTPProxy requests by checking
whether the request host matches the configured
ContainerWebServices.ExternalURL wildcard (previously it just checked
whether the request host looked like {uuid}-{port}.*, since there was
no ContainerWebServices config entry yet).

This enables the backend ContainerHTTPProxy method to interpret the
target as {uuid}-{port} or (in the near future) a published port name.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>