cli: make nicer report message for run-example command

[VMois]

closes #641

[codecov-commenter]

Codecov Report

Merging #642 (7ded734) into master (a8a94a1) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #642      +/-   ##
==========================================
- Coverage   18.88%   18.84%   -0.04%     
==========================================
  Files          26       26              
  Lines        2134     2138       +4     
==========================================
  Hits          403      403              
- Misses       1731     1735       +4     

Impacted Files	Coverage Δ
reana/reana_dev/run.py	26.63% <0.00%> (-0.48%)	⬇️

[mdonadoni]

LGTM! Only one minor comment to address

[mdonadoni]

(minor) What do you think about using _format_report when printing the test matrix? That way, if we want to modify the format in the future, we only have to change the code in one place for both the test matrix and the test results.

reana/reana/reana_dev/run.py

Lines 487 to 489 in a8a94a1

	f" - {len(components)} demo example(s): {', '.join(components)}\n"
	f" - {len(workflow_engines)} workflow engine(s): {', '.join(workflow_engines)}\n"
	f" - {len(compute_backends)} compute backend(s): {', '.join(compute_backends)}"

[VMois]

I did. Although, _format_report was only created to keep : or not depending on the list size. So not sure how much it will benefit in _test_matrix. But why not.