[Breaking] Correct global YAML merge order to lowest (from highest).

This addresses a merge order bug-of-intention between the following
files:

* .kitchen.yml (project config)
* .kitchen.local.yml (local config)
* $HOME/.kitchen/config.yml (global config)

The intended behavior was that:

1. baseline common configuration can go in the global config
2. any colliding values in the project config "win" over the global
   config
3. any colliding values in the local config "win" over the project
   config (and consequently also the global config)

This commit restores the intended merge semantics.

Closes #524
Merge pull request #555 from test-kitchen/yaml-merge-precedence

[Breaking] Correct global YAML merge order to lowest (from highest).