[bug/qa] Lint errors are ignored

[nemesifier]

Lint errors are ignored:
https://github.com/openwisp/ansible-openwisp2/blob/master/molecule/local/molecule.yml#L9-L12

This was probably forgotten.

We need to move those checks from there to the run-qa-checks script, if there's no run-qa-checks script, we must add one like we have in other repos.
We also need to make sure failing QA checks do not stop the entire build as we did in other modules, eg: openwisp/openwisp-controller@5fff20c (double check the latest version of the file for the latest optimal configuration).

We must not ignore failures and we must fix all the linting issues that are currently affecting this role.