executor: generate snakemake reports

[mvidalgarcia]

Support report operational option to customize report filename

closes reanahub/reana-commons#284 closes reanahub/reana-commons#288

TODO:

• [x] Run pip-compile --output-file=requirements.txt setup.py once https://github.com/reanahub/reana-commons/pull/289 is merged.

To test:

1. Run any snakemake example
2. reana-client ls should show report.html in the workspace.
3. reana-client download -w myworkflow report.html
4. open report.html
5. Repeat the same steps modifying reana-snakemake.yaml to add report operational option to customise the report filename.

   diff --git a/reana-snakemake.yaml b/reana-snakemake.yaml
   index 49cb624..b359eca 100644
   --- a/reana-snakemake.yaml
   +++ b/reana-snakemake.yaml
   @@ -7,6 +7,8 @@ inputs:
    - workflow/snakemake
   parameters:
    input: workflow/snakemake/inputs.yaml
   +  options:
   +    report: myreport.html
   workflow:
   type: snakemake
   file: workflow/snakemake/Snakefile

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