Enable configuration of the proxy from jenkins-puppet (take 2)

This allows for automation around setting the plugin proxy configuration.

For my needs, I only enabled setting the host and port, but this could be enhanced down the road to include user/pass.

As requested by @rtyler in #21, this is a rebased version. I could not figure out how to make the old merge request work, so I am just submitting this anew.

Thanks,
Dennis