Path-based routing returns 503 when a host has no root app

[trbernstein]

When using path-based routing with proxy.path_prefix, the proxy returns a generic 503 error if any configured proxy.hosts entry does not have an app deployed at /.

In our case, this occurred because the first host in proxy.hosts had no app handling the root path.

Expected behavior: hosts with a root app deployed should continue to work, even if other configured hosts do not have one.