concurrent_schema_changes_test snapshot_test dtest needs to account for hashed data dirs in 2.1

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ERROR: snapshot_test (concurrent_schema_changes_test.TestConcurrentSchemaChanges)
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Traceback (most recent call last):
  File "/home/mshuler/git/cassandra-dtest/concurrent_schema_changes_test.py", line 299, in snapshot_test
    for f in os.listdir(dirr):
OSError: [Errno 2] No such file or directory: '/tmp/dtest-VZwotc/test/node1/data/ks_ns2/cf_ns2'