tests: reduce osd-crush.sh execution time

A loop creating 255 crush rulesets via the command line takes
minutes. It is replaced by the generation of a crushmap instead.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2341 from dachary/wip-osd-crush

tests: reduce osd-crush.sh execution time

Reviewed-by: Sage Weil <sage@redhat.com>