cluster_info: Use terse buckets info

Only terse buckets info URI can provide correct ordering
in the nodelist wrt the vbmap serverList.

Also made GetBucket() to prevent doing any fetch operation.
We expect only cluster_info.Fetch() to do any network operations
and other APIs to consume whatver Fetch() and build.

Change-Id: I2220a505583675ab7fc91c38ed06479bec93a31b
Reviewed-on: http://review.couchbase.org/44171
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>