mailjet_host variable must be exposed in admin

I follow the installation instruction as described on the project page, fill my settings, and try to send a test mail.

I got the following error from phpmailer: "SMTP Error: Could not connect to SMTP host.". It's because the mailjet_host variable is empty.
The mail is sent when forcing mailjet_host to "in.mailjet.com" in mailjet.mail.inc

I could make a path for that, but I don't know the better way to handle mailjet_host variable:
1/ set "in.mailjet.com" as the fallback value every time variable_get('mailjet_host') is called
2/ Add a "Mailjet host" field in the setting page