upgrade/parallel/stress-split: fix test

Upgrade the second host before restarting the final monitor.

Run the dumpling api tests, since we still have half the osds running
dumpling.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit ee3d2277118c72f37d9c1c62e44134b0d1eb03b8)