rgw_admin: add --min-rewrite-stripe-size for object rewrite

A new param to check whether the object has requires restriping,
checking whether a specific object stripe is bigger than the specified
size. By default it is set to 0, and in that case it'll always be
restriped. Having it set to 4M + 1 will make sure that only the objects
that weren't striped before (using default settings) will be restriped.

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