[repo] Don't crash on start when fs schema is missing some db schema entities This occurs usually while developping and we don't want systematically to rebuild the database to start the instance or run e.g. i18ninstance.

Let configuration option be overridden by environment variables Related to #13889793.

[tox] Add a dummy test-results.xml file in check-manifest and flake8 environments This is to work around Jenkins Junit plugin that does not apparently account for the absence of this file (though it accepts it to be empty).

[tox] Basic flake8 config