cli: getting logs from workflow engine

[anton-khodak]

If a workflow developer / user runs a workflow, he/she must be able to get logs from the workflow engine in case something goes wrong. It is also a necessary feature for running CWL conformance tests: reanahub/reana-workflow-engine-cwl#9. I think it can be a command like reana-client workflow logs that dumps stdout from the workflow-engine to the client (straight to client's stdout or into a log file).

/cc @tiborsimko @diegodelemos @hjhsalo

[tiborsimko]

Improving access to logs is now part of of the Internal-Consolidation sprint: reanahub/reana#55

[dinosk]

Closing as logs are available from the server in the /status endpoint now.