add erasure code workload to upgrade/firefly-x

copied from suites/rados/thrash/workloads/ec-readwrite.yaml

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #45 from dachary/wip-ec-readwrite-firefly-x

add erasure code workload to upgrade/firefly-x