Add the ability for users to download NodeJS from a mirror and install automatically.

This feature request stems from https://github.com/jenkinsci/nodejs-plugin/pull/39 and will be implemented on that branch.

This feature will:

1. Be it's own type of installer - "MirrorNodeJSInstaller" which would show up in the drop down for installers as "Install from mirror"

2. Have the same features as the NodeJSInstaller (global packages to install, force32Bit, version dropdown)

3. Able to accept credentials for the mirror if required by the user

4. Handles self-signed certs by allowing disabling of TLS verification for NodeJS downloads.