letsencrypt: Should handle case of multiple accounts

As seen in https://discuss.freedombox.org/t/cannot-obtain-lets-encrypt-certificate/543, if there are multiple Let's Encrypt accounts, then the action to obtain a certificate expects user input to select an account. The action doesn't provide this input, so it fails.

The account information is part of the backup data for letsencrypt. If it is restored on a different system, then there will be multiple accounts.

Proposals:

• The actions should handle case of multiple accounts by selecting the newest one.
• When restoring from backup, we may want to check for multiple accounts and delete one.