Only log yaml config once, at startup

CASSANDRA-6456 introduced a feature where the yaml is dumped in the log. At startup this is a nice feature, but I see that it’s actually triggered every time it handshakes with a node and fails to connect and the node happens to be a seed (see here). Calling DD.getseeds() calls the SeedProvider, and if you happen to use SimpleSeedProvider it will reload the yaml config, and once again dump it out to the log.

It's debatable if DD.getseeds() should trigger a reload (which I added in CASSANDRA-5459) or whether reloading the seeds should be a different method (it probably should), but we shouldn't keep logging the yaml config on every connection failure to a seed.