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.
 Add more options to specify AWS credentials

    * Allow not to specify the access and secret keys explicitly in order to
      fallback to environment variable settings or EC2 instance metadata.
    * If logstash is run on Amazon EC2 instances, the use of the instance
      metadata allows easier maintenance and better security because the
      access keys are not explicitly listed in the configuration and
      managed by AWS itself.
- fix log4j input variable name bug (#635, etc)
- some style fixes
- time must be in UTC (LOGSTASH-1295)
- Pipeline now calls teardown on all input plugins when shutdown is
  requested
- Make udp input plugin abort when shutdown signal is given.
  (LOGSTASH-1290)
Merge branch 'cannedAcl'
Merge remote-tracking branch 'upstream/master'