MB-100: cluster_setup (system tests)

- Consider case when num_buckets = 0

Change-Id: I9e66e9f100847ae37989c6685d4af47826af60e6
Reviewed-on: http://review.couchbase.org/25249
Reviewed-by: abhinav dangeti <abhinav.dangeti@gmail.com>
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>