refactor: use workflow status as enum instead of integers

[diegodelemos]

Currently we are sending the workflow status changes from the workflow engines as the integer which represents the status, see for example in RWE Serial. This is happening because the enum which contains this statuses is inside REANA-DB and the workflow engines do not have DB as a dependency.

Even though it works it could easily lead to errors and it is definitely less readable. Also it would be more clear when dealing with the state-machine for workflow statuses.

A possible solution would be to move these enums (for Workflows and for Jobs) to REANA-Commons.

[diegodelemos]

It would be great if we also change status enum real values from integer to strings. E.g.:

class WorkflowStatus(enum.Enum):
    """Enumeration of possible workflow statuses."""

-    created = 0
+    created = 'created'
...

This would:

• Make code more readable and avoid cases like rest: add job start and finish times #374 (comment)
• Make database queries/rows more user friendly/readable