identityd: Add a health check route to identity server.
identityd: Add a health check route to identity server.

As per veyron/release-issues#1968

Change-Id: I098756d56397e5234e056de653dece756f55e1e5

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref