Use a regular command instead of the puppet() command

In the switch from beaker-puppet to beaker_puppet_helpers this was
removed. Now it uses a regular shell command to achieve the same.

Author: ekohl

spec/acceptance/netboot_spec.rb

@@ -18,7 +18,7 @@
   end
 
   describe 'ensure tftp client is installed' do
-    on hosts, puppet('resource', 'package', 'tftp', 'ensure=installed')
+    on hosts, 'puppet resource package tftp ensure=installed'
   end
 
   describe command("echo get /grub2/grub.cfg /tmp/downloaded_file | tftp #{fact('fqdn')}") do