invalid validation warning for `kubernetes_uid` for serial workflows

[tiborsimko]

Current behaviour

Consider the following valid workflow definition:

workflow:
  type: serial
  specification:
    steps:
      - environment: "docker.io/library/python:2.7-slim"
        kubernetes_memory_limit: "712Mi"
        kubernetes_uid: 713
        commands:
          - id

The execution shows that the job is well executed as user 713:

$ reana-client run -w test
...
$ reana-client logs -w test | grep uid
 uid=713 gid=0(root) groups=0(root)

However, the validation raises a warning about kubernetes_uid:

$ $ reana-client validate
==> Verifying REANA specification file... reana.yaml
  -> WARNING: The REANA specification appears valid, but some warnings were found.
  -> WARNING: Unexpected properties found in REANA specification file: 'kubernetes_uid' (at workflow.specification.steps.0).
  -> WARNING: Please make sure that the REANA specification file is correct.
==> Verifying REANA specification parameters...
  -> WARNING: Workflow "inputs" are missing in the REANA specification.
==> Verifying workflow parameters and commands...
  -> SUCCESS: Workflow parameters and commands appear valid.
==> Verifying dangerous workflow operations...
  -> SUCCESS: Workflow operations appear valid.

Expected behaviour

The property kubernetes_uid should be recognised without warning, just like kubernetes_memory_limit was.

Notes

Seen with maint-0.9 branches.