It is now possible to set the user login (instead of
having it hardcoded to "docker"). Also, various actions
have been broken out in separate functions to facilitate
future maintenance.
Instead of upgrading from 1.16 to <latest> we upgrade from 1.15
to 1.16, because upgrading from <latest-1> is a special case and
it is better to show the general case.
Also, the script that sets up admin clusters now has some retry
logic to accommodate hiccups in pssh or in the cloud provider.
