Use local variables in functions

This will avoid undesired side-effects.
Make get_allocated_nodes always set get_allocated_nodes_retval as retval

The old code was broken because the name of the variable passed to
get_allocated_nodes to save the result should not be used inside the
function; but it was...
Fix log in get_allocated_nodes to mention role
Check the nodes have packages updated to SUSE Cloud 4 packages

We do that before the reboot (and the migration of neutron bits).

https://bugzilla.novell.com/show_bug.cgi?id=892500
Use xpath command instead of ugly xmllint command
fix test for user answering Y
fix post step error message