Export prepare_node variables rather than pass them around

The prepare_node scripts are primarily called from within nodepool,
which only passes in a single argument: the hostname of the node.

The rest of the values that we want to pass in to the underlying
prepare_node script can be shared through the environment, making order
no longer important and alleviating the need to duplicate setting
defaults in order to pad positional arguments to a newly created
variable.

Change-Id: I0a13e27524b75a50a482870414ebe2fd6ac8315e
Merge "Export prepare_node variables rather than pass them around"