We had another bug where we confused the hostname with the NodeName.

To avoid this happening again, and to make the code more
self-documenting, we use types.NodeName (a typedef alias for string)
whenever we are referring to the Node.Name.

A tedious but mechanical commit therefore, to change all uses of the
node name to use types.NodeName

Also clean up some of the (many) places where the NodeName is referred
to as a hostname (not true on AWS), or an instanceID (not true on GCE),
etc.