Only complain about the deprecation if the stated standards aren't met.

You can't tell people that by setting 'do_not_discover_peers' they can
specify 'hosts', and then not let them do that. ;)
Make sure tests can run.

Since someone might reasonably test against a single-host Cassandra
install, and that will return empty results from the system.peers
table, we should use do_not_discover_peers to make sure the tests at
least have a chance to connect.