rgw_user: Create RGWUser class.

Clean up the existing user operations in order to support a RESTful user
management api.

Signed-off-by: caleb miles <caleb.miles@inktank.com>
rgw_user: Add user sub-resources to the heap and consolidate operations.

Do not dynamically allocate RGWUser helper objects.

Signed-off-by: caleb miles <caleb.miles@inktank.com>
rgw_user: Add caps operation wrappers.

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