An S3 Bucket owner may wish to set a canned ACL (as opposite to explicitly set
grantees, etc.) for an object. This commit adds an optional "acl" attribute to
the aws_s3_bucket_object resource so that the owner of the S3 bucket can
specify an appropriate pre-defined ACL to use when creating an object.
Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>