Plugin installs now working

I have added some sleeps to the commands to give them time for Jenkins to finish booting as the install-plugin command requires the GUI to be up and running.  Also, there is a strange thing whereby the plugin can't be resolved by its name alone.  I tried to solve this by referencing the URL directly but this only worked if I specified an exact version - if you use the permalink it resulted in just getting the redirect request back to the latest version instead of the plugin itself.  Finally, I found a solution that involves grabbing the update centre JSON and firing it at the local Jenkins so that the plugins can be resolved by name alone.  The jenkins-cli command can take multiple plugins on one line but I've kept them in a loop running one-at-a-time for now as this is the first time everything is working ok.