[bug] Update auto-install script to prompt yes/no for Let's Encrypt instead of email

[pandafy]

Currently, the auto-install script prompts users for an email address for Let's Encrypt certificates:

Enter letsencrypt email (leave blank for self-signed certificate)

As Let's Encrypt no longer sends renewal emails (announcement), collecting an email is no longer necessary to complete certificate issuance.

Tasks:

• Update the auto-install script to prompt a simple yes/no question when setting up OpenWISP, instead of requesting an email address.
• Adjust the certbot command and any logic depending on CERT_ADMIN_EMAIL as needed.
• Remove, update, or refactor related email documentation, variables, and script logic.
• Ensure self-signed certificate flow still works when Let's Encrypt is skipped.
• Update comments and documentation as appropriate.

Reference: https://letsencrypt.org/2025/01/22/ending-expiration-emails

[pranshustuff]

Hi, I'd like to work on this issue.