[Sidekiq] sidekiq::cleanup is invoked every chef run

https://github.com/engineyard/ey-cloud-recipes/blob/master/cookbooks/sidekiq/recipes/default.rb#L6

cleanup needs to run only when changing the instance(s) where sidekiq is run:
- run sidekiq on utility instance instead of running on app instances
- change the utility instance name where sidekiq is run

Basically, when this part of the recipe is changed: https://github.com/engineyard/ey-cloud-recipes/blob/master/cookbooks/sidekiq/recipes/setup.rb#L6

In practice this very rarely happens. For most cases, and especially on production environments, after the strategy (run on application or utility instances) has been set, cleanup should not be run.

Changes needed:
- Comment out line 6 in recipes/default.rb
- Update the README - mention that cleanup is disabled by default. Discuss the consequences: if changing the sidekiq utility name, cleanup should be run or manual cleanup should be done.