rgw: bucket index ops on system buckets shouldn't do anything

Fixes: #4508
Backport: bobtail
On certain bucket index operations we didn't check whether
the bucket was a system bucket, which caused the operations
to fail. This triggered an error message on bucket removal
operations.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit 70e0ee8ba955322832f0c366537ddf7a0288761e)