Gitlab plugin notebook may fail nbval test

[dmfigol]

docs/plugins/tasks/version_control/gitlab.ipynb
Problematic cell:

result = n.run(
    gitlab,
    action="get",
    url="http://localhost:8080",
    token="SuperSecretToken",
    repository="test",
    ref="master",
    filename="hosts",
    destination="/tmp/hosts"
)

This cell output test will succeed only if the file /tmp/hosts does not exist already.