serial: add tests to partial execution

[diegodelemos]

• Closes #88.

[tiborsimko]

Running some tests right now... In the meantime you can perhaps enrich docs/userguide.rst with a new section on partial workflow execution.

Can be simple, something like:

Partial workflow execution
----------------------------------

Typically you workflow contains of several steps, for example gendata and fitdata.  You can name them like this:

... # excert from reana.yaml

If you would like to run the serial workflow only up to a certain step, you can specify operational option TARGET when starting the workflow:

... # reana-client start -o TARGET=gendata

which will run only the generation data step.

This can be used for debugging parts of workflows faster.

[tiborsimko]

Two observations:

• -o TARGET=gendata does not work, one has to use lowercase -o target=gendata. It may be good to accept uppercase as per analogy with -o CACHE=off.

• The partial workflow execution seems to work well, i.e. only the given step is run and also the status reporting counts only needed commands, but the pod is "hanging" in runtime state, as it were, while it could finish:

$ reana-client ls -w partial
NAME                SIZE     LAST-MODIFIED
gendata.log         2145     2019-09-12T09:13:28
results/data.root   153073   2019-09-12T09:13:28
code/fitdata.C      1648     2019-09-12T09:12:54
code/gendata.C      1937     2019-09-12T09:12:54
$ reana-client status -w partial
NAME      RUN_NUMBER   CREATED               STATUS    PROGRESS
partial   2            2019-09-12T09:12:49   running   2/2
$ kubectl get pods | head -2
NAME                                                      READY   STATUS    RESTARTS   AGE
batch-serial-802d98d6-ff9a-43d2-bdf4-842e6a649a51-zffbl   1/2     Running   0          5m15s
$ kubectl logs batch-serial-802d98d6-ff9a-43d2-bdf4-842e6a649a51-zffbl workflow-engine
/usr/local/lib/python3.6/site-packages/bravado_core/spec.py:292: Warning: uuid format is not registered with bravado-core!
  category=Warning,