Support ssh access to Mercurial repository

Problem

HTTPS access always ask for a password, and automated processes need to be able to identify with an SSH key.

Current version of heptapod allows ssh access to the Git repository, but writing to it is discouraged, and this ability will be disabled by #17 (closed) as we don't know what will be broken if we do git changes and push them via git.

Task description

Enable the SSH gateway towards the Mercurial repository