Add an option (--low-compression) to cbworkloadgen so that we can
create workloads that will use disk and memory almost equally.

Patch was in my sandbox and had been used for creating datasets that
utilised disk heavily.

Change-Id: I892d5b508467568ec738f0751cab5640ab52a87d
Reviewed-on: http://review.couchbase.org/42000
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>