Startup code needs to know the dc/rack of the local node early, way
before nodes starts any communication with the ring. This information is
available when snitch activates, but it starts _after_ token-metadata,
so the only way to put local dc/rack in topology is via a startup-time
special API call. This new init_local_endpoint() is temporary and will
be removed later in this set
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>