ansible how to set a default ssh user for all hosts?

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 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?

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