[feature] Consent mechanism for clean insisghts statistics

[nemesifier]

Depends on openwisp/openwisp-utils#360.

We shall add a task which asks confirmation to the user to allow us to track some basic statistics which help us with applying for Open Source funding.
We will need to clarify what statistics we collect (we can link to the README section of openwisp-utils added in openwisp/openwisp-utils#360).

[pandafy]

@nemesifier vars_prompt in play and not in ansible roles

If you want your playbook to prompt the user for certain input, add a ‘vars_prompt’ section. Prompting the user for variables lets you avoid recording sensitive data like passwords. In addition to security, prompts support flexibility. For example, if you use one playbook across multiple software releases, you could prompt for the particular release version.

An alternative solution would be to show the message that the ansible role enabled clean insights measurement collection and user's can turn it off by setting a variable. We can have a timeout of 30 seconds for users.