Ansible version 2.1

I have an inventory file `hosts`

```
[nodes]
host1
host2
...

```

And a simple playbook `site.yml`

```
---
- hosts: all
  tasks:
    - include: tasks/main.yml

```

If I just start the play,

```
ansible-playbook -i hosts site.yml -vvvv

```

I get this error for all hosts,

```
ESTABLISH SSH CONNECTION FOR USER: None
fatal: [host1]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh.", "unreachable": true}
...

```

However, reading [Ansible Inventory](http://docs.ansible.com/ansible/intro_inventory.html#list-of-behavioral-inventory-parameters) doc, I added `ansible_user` to the `hosts` file,

```
[nodes]
host1    ansible_user=root
host2    ansible_user=root
...

```

This solves the `SSH CONNECTION UNREACHABLE` error. However, do I have to add `ansible_user=root` next to all the hosts? Or is there a simpler way to do this?