Install and configure Dashing with Icinga 2 to use the external board for your Icinga2 instance.
Upgrade icinga2 and its web-interface using official/dedicated repository and PostgreSQL database.
Create custom data field to define multiple domain names and service apply rule to verify those SSL certificates.
Icinga2 documentation clearly describes the master->satellite->client setup, but as of now everything can be configured using director module and top down approach, so you can easily monitor external remote networks that are not accessible from the master server.
Icinga is a great monitoring solution, especially when you use director module to perform configuration in top down environment. It makes application more accessible to more people as you can use web-interface and role based permissions to allow developers and testers to write and use their own checks and notifications.
Install Icinga 2 monitoring instance with web-interface.