How to find which is the review board site directory from inside my extension

[Kartik]

Hello,

I am trying my hand at writing an extension that uses signal hooks (e.g. on_review_published) to update the jira ticket with the progress of the code review. I would like to read the jira server access info from a settings file, and the site directory (default: /var/www/reviewboard) is a good place for that file. Rather than hard-coding the path, I'd like to find what is the site directory folder, and look in there. 

My question is: How can I find which is the site directory folder from inside my extension? 

Thanks!

Kartik