Set default --batch-size in client v2 to 1

The default --batch-size value in client v2 is set to 5 [1], which may be too high for a small job to keep up with its SLA. The default of 1 should be a sane alternative.

[1] - https://github.com/apache/incubator-aurora/blob/master/src/main/python/apache/aurora/client/cli/options.py#L199-L200