MAR-218: Use offline mode when running ansible-lint:

- Avoids calling out to remote servers (and leaking info about
caller).

MAR-218: Disable Ansible lints for prefixed role variable names:

- At this point in time, it does not make much sense to introduce
prefixing and breaking every single role invocation.
- Might be worth revisting down the line, although it would require
understanding a bit more clearly if prefixing has any kind of
special logic handling or it's pure guidelines.