always use root for ansible_user

while the user is currently not exposed via the api (see [1]), it's
always `root` and inventory consumers should not have to set this
themself

[1] CentOS/duffy#608

plugins/inventory/inventory.py

@@ -75,6 +75,8 @@ def parse(self, inventory, loader, path, cache=True):
                 self.inventory.add_host(node.hostname)
 
                 self.inventory.set_variable(node.hostname, 'ansible_host', str(node.ipaddr))
+                # always use `root` until https://github.com/CentOS/duffy/issues/608 is fixed
+                self.inventory.set_variable(node.hostname, 'ansible_user', 'root')
 
                 host_vars = {'duffy_session': session.id}
                 for key, value in dict(node).items():