MB-1143 Rack-aware vbucket map generation (replica count > 1).

Current limitations:

  - best effort is still not there
  - the process of generating matrix R from RI is still not rack
    aware; so for certain RIs even if fully rack aware R is feasible,
    the generated one might not be fully rack aware; and hence the
    induced vbucket map will have small number of vbuckets for which
    rack constraints are violated

Change-Id: Id17c5ab4bacb4e3dcb53e99250004bce41dd8413
Reviewed-on: http://review.couchbase.org/30276
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>