MB-1143 Update vbmap.

Change-Id: I6a414c9b68bcb92843265a6396599d39eedf12f5
Reviewed-on: http://review.couchbase.org/29614
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-1143 Rack-aware vbucket map generation (replica count = 1).

Besides the limitation on number of replicas, if rack-aware vbucket
map couldn't be generated, then we fallback to rack-unaware vbucket
map generator. Eventually vbmap tool will learn how to find
approximate solutions.

Change-Id: I537d0987b147c75f5f93f7585595a67d26213fec
Reviewed-on: http://review.couchbase.org/29615
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>