MB-10164: Set numShards based on disk type

In configuration.json, numShards is initialized to zero.
If this value is greater than zero, this will be used
as numShards, otherwise using sigar we determine the
disk access, based on which numShards gets set to either
4 or 2. If sigar also fails, set numShards to 4 by
default.

SSDs: numShards=4, HDDs: numShards=2

Change-Id: Ie79348816fbd4bc11648c825014e6670b451bd99
Reviewed-on: http://review.couchbase.org/34744
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>