CBQE-0: _get_bucket_size without ratio

we twice underestimate memory for buckets:
1) in NodeInitializeTask  self.quota = int(info.mcdMemoryReserved * 2 / 3)
https://github.com/couchbase/testrunner/blob/master/lib/tasks/task.py#L121
2) when we create bucket (_get_bucket_size)

for example: if we are able to initialize cluster/node with 1000M then bucket will be created only with 444MB
1000*2/3*2/3

Change-Id: I60a7a502d4630bfbb635a02c2d0c7e96b6536371
Reviewed-on: http://review.couchbase.org/35350
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>