Revert "Revert "rgw: if extra data pool name is empty, use data pool name instead""

This reverts commit 0b6bd2545925b5e8a80d41de1fda13ffe9d30e2b.

We confused commit 5fd8b0d1639c67e355f0fc0d7e6d7036618d87a1 with commit
b1a4a7cb91e164d1f8af8ce9319e3b3c1949858d in our tests.  We tested without
the latter, saw a failure, applied it and then reverted the former, and it
passed, but didn't actually resolve the problem.

This puts them both back in place and all should be well.

Signed-off-by: Sage Weil <sage@redhat.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>