acl: add preliminary support for S3 ACL

Currently, Swift3 sets and retrieves Swift ACLs for S3 ACL requests.  However,
S3 ACL is too different from Swift ACL to implement the below reference.
  http://docs.aws.amazon.com/AmazonS3/latest/dev/S3_ACLs_UsingACLs.html
With this patch, Swift3 uses its own metadata for ACL
(e.g. X-Container-Sysmeta-Swift3-Acl) to achieve the best S3 compatibility.
This patch only embeds the S3 ACL into the Swift metadata.  The swift3
middleware does not use it for S3 requests yet; this will be addressed later.

Change-Id: I4522910b6b3a0066f24caa98727fdeb85837e42b