rgw_acl: Support ACL grants in headers.

Issue 3669: Support S3 ACL grants specified in request headers. Allow
requests, excluding POST object, to specify ACL grants in HTTP headers.

Signed-off-by: caleb miles <caleb.miles@inktank.com>

Conflicts:
	src/rgw/rgw_acl_s3.cc
	src/rgw/rgw_acl_s3.h
	src/rgw/rgw_rest_s3.cc
	src/rgw/rgw_rest_s3.h

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: refactor header grants

Move definition to a static array.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>