Currently, it is not possible to specify a username and password/identity file to use for checking out a Git-based project. This is highly inconvenient as it relies on the fact that the user used to checkout the project will be the same as the one running Jenkins, or that you have a ~/.ssh/config rule for that host.

When using a username in the git URL, you are not asked for a password and Jenkins fails to authenticate.

Ideally, this could be handled via the web interface in Jenkins.