CLI: generate OpenAPI specs for generic components

[diegodelemos]

Stemmed from #6. We should centralize the command to generate specs which, right now, is replicated in different components, i.e. here.

Depends on reanahub/reana-job-controller#106, since the decision taken influences this initialisation. For instance, if we decide to use marshmallow we should be able to dynamically provide marshmallow models to the OpenAPI generator, i.e.:

def build_openapi_spec(publish, marshmallow_schemas):
    ...
    for name, schema in mashmallow_schemas:
        spec.definition(name, schema=schema)

[diegodelemos]

Add title, version as parameters in build_openapi_spec() in spec.py

Stemmed from reanahub/reana-job-controller#86.

[diegodelemos]

Add openapi spec in get_openapi_spec() in app.py

Stemmed from reanahub/reana-job-controller#85