admin: show hostname in clusters output (#7362)

Part of https://github.com/envoyproxy/envoy/issues/1606

Risk Level: Low
Testing: Modified UT
Docs Changes: N/A
Release Notes: Added

Signed-off-by: Matt Klein <mklein@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ 7816d5e4baa1dee7558572393c83fa1d96a5052a

envoy/admin/v2alpha/clusters.proto

@@ -65,6 +65,9 @@ message HostStatus {
 
   // The host's weight. If not configured, the value defaults to 1.
   uint32 weight = 5;
+
+  // The hostname of the host, if applicable.
+  string hostname = 6;
 }
 
 // Health status for a host.