This uses eatmydata with zookeeper when we run our tests. This
should allow us to avoid using a tmpfs (which uses memory) for
ZK, which generally shouldn't need to read back any data in our
unit tests, so we don't really care how long background writes
take. This should save 3/4 of a GB of ram.
Change-Id: I64ef1e9028d836c91d1c7c59909ac1f5a54db2a5
3ee8a4f082