cli: configurable outputs for run-example

[tiborsimko]

Developers can use reana CLI to test all workflow engines for a particular example via one command:

$ cd reana-demo-worldpopulation
$ reana run-example -c .
...
[reana-demo-worldpopulation] reana-client list -w worldpopulation.yadage | grep -q plot.png
[reana-demo-worldpopulation] echo OK
OK

The command tests for the presence of plot.png in the output.

However some examples such us reana-demo-helloworld produce different output files, e.g. greetings.txt.

We should make this configurable, for example introduce a new configuration variable in the beginning of the script:

EXAMPLE_OUTPUTS = {
   'reana-demo-helloworld': ('greetings.txt`,),
   '*': ('plot.png`,)
}

and amend run_example() to test for given files.

[tiborsimko]

Implemented in #104.