attributes to custom the owner/group

As the filename location can be changed with an attribute, it is desirable to
make the owner and group customizable, since someone may wish to use a
different user than root for AWS access.

For example, a sensu plugin may need to read the credentials when running a
plugin as the `sensu` user. A Jenkins server may need to upload files to an S3
bucket as the `jenkins` user, etc.