serial: add kubernetes_uid field

[alintulu]

Add kubernetes_uid field to serial workflow to allow user to specify the Kubernetes runtime user ID

closes reanahub/reana-workflow-engine-serial#106

[diegodelemos]

Works, usage with REANA-demo-helloworld:

1. Modify reana.yaml:

   diff --git a/reana.yaml b/reana.yaml
   index 50d1bd8..881ac3b 100644
   --- a/reana.yaml
   +++ b/reana.yaml
   @@ -13,6 +13,7 @@ workflow:
   specification:
    steps:
      - environment: 'python:2.7-slim'
   +        kubernetes_uid: 1001
        commands:
          - python "${helloworld}"
              --inputfile "${inputfile}"

2. Run:

   $ reana-client run