node lists support implemented

Idea of this patch is to create special file, which denotes list of nodes, and by design,
reclass assumes, that .yml file with the same name exists. thats gives us several
identical nodes, which names are listed in .hosts file

example: (also example available in examples directory)

we can create file group1.hosts and group1.yml, thats gives us effect of several identical .yml
files with the same content as group1.yml.

as for performance, seems that it works reasonably fast, event for 4k hosts.