MB-12670: Increase default number of vbucket per process

Change-Id: I449ca7eded4a6729c6348db16cad3a240fc05a7a
Reviewed-on: http://review.couchbase.org/43324
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>