Even though the GIT SCM module has the ability to provide "credentials" (SSH key) for the repository, the git-parameter plugin doesn't seem to use them. The only way we have been able to get it to work is to configure the jenkins user (at the shell) to have a passwordless SSH key with access to the repo. Once the "build" starts, it appears to properly setup the ssh-agent, and further repository access works using the correct credentials.