Use s3 asset caching only when we want it.

omnibus.rb

-use_s3_caching true
+use_s3_caching ENV['USE_S3_CACHE'] || false
 s3_access_key ENV['CACHE_AWS_ACCESS_KEY_ID']
 s3_secret_key ENV['CACHE_AWS_SECRET_ACCESS_KEY']
 s3_bucket ENV['CACHE_AWS_BUCKET']