[ci] Avoid duplication of basic running test

Created by: nemesisdesign

These lines seem duplicated:

• https://github.com/openwisp/ansible-openwisp2/blob/master/.travis.yml#L31
• https://github.com/openwisp/ansible-openwisp2/blob/master/.gitlab-ci.yml#L16-L19

Maybe we can extract those lines in a script that gets called by both CI systems