rgw: gracefully handle bad root pool names

Fixes: #5716
When invalid root pools specified (don't start with a period) we return
with an error instead of asserting.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #729 from ceph/wip-5716

rgw: gracefully handle bad root pool names

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>