rgw: mark ec related rgw suites as slow backend

We want to avoid tests that time out due to a too slow backend.

Reviwed-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit 85221bc3258b27f0a6b86a74aa1734dac16afe1d)