introduce --viz flag to enable/disable Yadage workflow visualization

[VMois]

In cli.py we have an option to visualize Yadage workflow at the end of the execution. It is True by default rn. We can introduce an option to enable/disable it as an engine-specific config in reana.yaml (I guess).

This can be useful when 10k+ workflows are run as it will generate a couple of GBs of GIFs, and if workflows are the same it is not that useful anymore.

[lukasheinrich]

providing a generic mechanism to pass engine-options would be useful