puppetize installation of gerrit third party plugins

This change automates the installation of gerrit third party plugins.  This
does not include core plugins, which have already been automated.  The first
third party plugin that we automate is the javamelody plugin.

Change-Id: I163353c2e9d59e4815b4544f983b54d68ede70a8
Merge "puppetize installation of gerrit third party plugins"