veyron/lib/flags: provide a default value for veyron.namespace.roots
veyron/lib/flags: provide a default value for veyron.namespace.roots

Change-Id: I3a809785c4fa64c8f33fe2fd7a9ca7a2822ea53d

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23