puppet: support forcing plugin installation allowing us to install latest git-client plugin

Related to commit c73c341f ("puppet: install git-client plugin
explicitely in version 1.1.2"), instead of pinning the git-client plugin
to a specific version also install the latest versions of the ssh-agent
and credentials plugins and pin the credentials plugin so it doesn't
get restored/downgraded by Jenkins service itself.

Thanks stephenconnolly for the hint regarding the pinning trick.
See https://issues.jenkins-ci.org/browse/JENKINS-19927