Refactoring xhost.rb to be clearer

It's a simple tool for generating a host dependency graph,
and that wasn't clear at all.

Signed-off-by: Luke Kanies <luke@puppetlabs.com>
Renamed bin/xhost.rb to bin/show_host_dependencies

It's a more meaningful name.

Signed-off-by: Luke Kanies <luke@puppetlabs.com>