Refactored grains cache loader.
Refactored grains cache loader.

  - refresh_grains_cache opt will now give a distinct debug message
  - if the grains fail to load from cache, or if the cache is vaild but is
    empty, grains will be reloaded
  - dramatically reduced nesting by extracting loader to dedicated function and
    inverting the condition clauses.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt