erasure-code: enable ec-rados-default.yaml

erasure-code: enable ec-rados-default.yaml

The ec-rados-default.yaml started with:

  workload:
    sequential:

which is only suitable for suites/upgrade/giant-x/parallel/2-workload/sequential_run/ec-rados-default.yaml

because suites/upgrade/giant-x/parallel/1-giant-install/giant.yaml has

   - parallel:
      - workload
      - upgrade-sequence

The same file was included in contexts where the parallel task was not
used and the workload did not run:

  ./suites/upgrade/firefly-x/stress-split-erasure-code/5-workload/ec-rados-default.yaml
  ./suites/upgrade/giant-x/stress-split-erasure-code/5-workload/ec-rados-default.yaml
  ./suites/upgrade/giant-x/stress-split-erasure-code-x86_64/5-workload/ec-rados-default.yaml

The ec-rados-default.yaml is modified to be a task instead of a
sequential task in a parallel tasks. The ec-rados-sequential.yaml is
added and is linked in
suites/upgrade/giant-x/parallel/2-workload/sequential_run instead of ec-rados-default.yaml.

http://tracker.ceph.com/issues/11221 Fixes: #11221

Signed-off-by: Loic Dachary <loic@dachary.org>