Allow PALLET_HOSTS to set node-file for node-list

Allow the use of the PALLET+HOSTS environment variable to set the
location of the default node-file used when constructing a node-list
service.
Refactor node-list functions
Change node to take hostname

The ip and group defaults to the hostname.
Resolve node-list hostnames on creation
Allow map of group to nodes in node-file

Allow groups to be specified as a map in the node-file.
Add argument checking to node constructor