add rack permissions at the installation plugin

base on puppet_external_nodes/blob/master/install.php file

it's add 3 permission at the plugin installation:
rack_add permit to see and add rack (include rack assignment)
rack_mod permit modify rack (include rack assignment)
rack_del permit to delete (rack (include assignment)

i have try with the last upgrade of rack_maint, the installation creat the permissions and it's work