How is the repo cloned on builder slave?

[xcompass]

I'm trying to make this plugin work on external jenkins. I got most parts. (So close to send a pull request!) When I test it on a private repo, such as openshift repo (ssh://5331074d606b939f17000001@mytest-compass.example.com/~/git/mytest.git), I got permission denied error message. It seems I don't have correct private key to to access the repo. However, the unmodified plugin works fine. I couldn't figure out how the private key is specified. Could someone point me to the right direction? Thanks.

Here is the error message I got with modified plugin:

Wiping out workspace first.
Cloning the remote Git repository
Cloning repository origin
ERROR: Error cloning remote repo 'origin' : Could not clone ssh://5331074d606b939f17000001@mytest-compass.example.com/~/git/mytest.git
ERROR: Cause: Error performing command: git clone --progress -o origin ssh://5331074d606b939f17000001@mytest-compass.example.com/~/git/mytest.git /var/lib/openshift/533f2df0606b93525a00022a/app-root/runtime/repo
Command "git clone --progress -o origin ssh://5331074d606b939f17000001@mytest-compass.example.com/~/git/mytest.git /var/lib/openshift/533f2df0606b93525a00022a/app-root/runtime/repo" returned status code 128: Initialized empty Git repository in /var/lib/openshift/533f2df0606b93525a00022a/app-root/runtime/repo/.git/
Warning: Permanently added 'mytest-compass.example.com,xxx.xxx.xxx.xxx' (RSA) to the list of known hosts.
Permission denied, please try again.
Permission denied, please try again.
Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).
fatal: The remote end hung up unexpectedly