shell() expects node to be specified with argument :n, not :d

puppet_module_install helper fails when attempting to copy a puppet module to a non-default node (in multi-node test environments).

The helper creates a directory for the module via shell("mkdir...") before using scp to copy module files. Currently it passes the node  using wrong parameter (passing `:d` instead of `:n` - see RSpecSystem::Helpers::Shell#L34](https://github.com/puppetlabs/rspec-system/blob/master/lib/rspec-system/helpers/shell.rb#L34)), so this command is always executed on the default node and the following scp() fails since the directory does not exist.