How to auto-yes "drush features-revert-all" ... via terminus
You need to add a -- separator to your command. The options for Terminus come before this marker, and the options for Drush come after.

In other words:

terminus -y drush site.dev -- features-revert-all -y


Explanation:

In programs that support it, -- means "all of the options that come after should be treated as arguements". The command terminus drush passes all of its arguments through to Drush.