tests: Making "objectstore" calls parallel in osd-scrub-repair.sh

tests: Making "objectstore" calls parallel in osd-scrub-repair.sh

osd-scrub-repair is making several similar objectore calls in a
sequential way while they could be easily parallelized.

Each single objectore call can spent up to dozen of seconds so making
the call parallel is saving a lot of time while keeping the code pretty
simple.

This particular patch saves approx. 2 minutes on the actual code on a recent
laptop. The global running time of osd-scrub-repair drops from 9m33 to
7m37 !
Signed-off-by: Erwan Velu <erwan@redhat.com>