Cluster: Fix trib create when masters==replicas

This bug was introduced in 2e5c394f during a refactor.

It took me a while to understand what was going on with
the code, so I've refactored it further by:
  - Replacing boolean values with meaningful symbols
  - Replacing 'i' with a meaningful variable name
  - Adding the proper abort check
  - Factoring out now duplicated conditionals
  - Adding optional verbose logging (we're inside *four*
    different looping constructs, so it takes a while to
    figure out where all the moving parts are)
  - Updating comment for the section

This fixes a problem when the number of master instances
equaled the number of replica instances.  Before, when
there were equal numbers of both, nodes_count would go to
zero, but the while loop would spin in i < @replicas because
i would never be updated (because the nodes_list of each ip
was length == 0, which triggered an endless loop of
next -> i = 0 -> 0 < 1? -> true -> next -> i = 0 ...)

Thanks to carlo who found this problem at:
https://groups.google.com/forum/#!topic/redis-db/_WVVqDw5B7c