First (rough) implementation of a cloud3->cloud4 upgrade

This introduces five new mkcloud commands for testing the different steps of
the cloud3 to cloud4 upgrade as outlined in the manual:

"prepare_cloud4upgrade":
    Sets up the Cloud 4 repos on the admin server and
    installs the latest suse-cloud-upgrade code from a git checkout.

"cloud4upgrade_1st":
    Runs suse_cloud_upgrade upgrade for the first time (the pre step)

"cloud4upgrade_2nd":
    Updates packages on the admin node and runs suse_cloud_upgrade upgrade for
    the second time (post step)

"cloud4upgrade_clients":
    Deploys the updater barclamp on all client nodes to install the cloud 4
    packages

"cloud4upgrade_reboot_and_redeploy_clients":
    Final step. Calls suse_cloud_upgrade reboot-nodes and redeploys all
    barclamps with exisiting proposals on the clients

This currently needs a mkcloud deployed Cloud3 with "cloudsource=GM3" and
"TESTHEAD=1" to have all maintenance updates installed.
You can use the `proposal_set_value` function to do this magic for you.