ceph_test_librados_tier: tolerage EAGAIN from pg scrub command

We may get EAGAIN if the osd happens to be down, for example due to
thrashing.  Try a few times and then give up.

Note that the other place we try to scrub we don't even check the return
value as we are poking ever pg in the pool.  And the scrub commands get
lost due to any peering event, etc.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1657 from ceph/wip-8063

ceph_test_rados_api_tier: fix scrub test

Reviewed-by: Samuel Just <sam.just@inktank.com>