MB-1143 Rack-aware vbucket map generation updates.

  1. Multi-replica case is handled much better now: fully rack-aware
  vbucket map is generated almost always when it's possible.
  2. Best effort. Few things are tried in order:

     a. change number of slaves
     b. sacrifice rack awareness for replicas (master is still
     guaranteed to be on a different rack)
     c. sacrifice rack awareness for master too
     d. sacrifice replica vbuckets balance

Change-Id: I6daa85a26d54ab65e4bb2f0c3a0e0dc35a0d34cb
Reviewed-on: http://review.couchbase.org/30789
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>