vagrant 1.8.1 não parece instalar o ansible quando é especificado em ansible_local

Meu provisionador está assim:

config.vm.provision :ansible_local do |ansible|
  ansible.playbook = '/home/deploy/sosd/local/ansible/sosd.yml'
  ansible.install = true
end

Mas vagrant upparece que não instala:

==> default: Running provisioner: ansible_local...
    default: Installing Ansible...
The Ansible software could not be found! Please verify
that Ansible is correctly installed on your guest system.

If you haven't installed Ansible yet, please install Ansible
on your Vagrant basebox, or enable the automated setup with the
`install` option of this provisioner. Please check
https://docs.vagrantup.com/v2/provisioning/ansible_local.html
for more information.

O que estou fazendo errado?