Add option to specify canned ACL in AWS S3 output configuration

    * Adds the option "canned_acl" which defaults to "private"
    * Breaks the previous behaviour where the canned ACL was hard-coded to "public_read".
      Which is a potential security problem, allowing everyone to access the file on S3.
Merge pull request #637 from hduregger/cannedAcl

Add option to specify canned ACL in AWS S3 output configuration