Add ssh-credentials support

Hi, I've cobbled the support for ssh-credentials. What I hoped for was to make it optional, so it doesn't interfere with older Jenkins installs that cannot, or simply do not use the credentials plugin. I would like to work on this further by contributing to the credentials plugin so that the credentials select Jelly taglib has the name attribute, so that the ugly workaround I employed here could be removed.

Could you please review and see if this is the way to go about adding ssh-credentials support, and if indeed I've managed to make it truly optional.

Thanks ;)