With rf_rack_valid_keyspaces enabled, RF of alternator tables will be
equal to the number of racks (in this test: nodes). Prior to that, if
number of nodes is smaller than 3, alternator creates the keyspace
with RF=1. Turns out, with RF=2 the test fails with write timeouts due
to contention. Enforce RF=1 by creating the table with one node before
adding the second node.