ansible thinks /etc/ansible/host_vars/localhost format is json

Charms that use ansible give this error:

2014-05-09 19:08:00 INFO install fatal: [localhost] => /etc/ansible/host_vars/localhost: Expecting property name: line 1 column 1 (char 1)

As with Ansible version 1.6, Ansible add support for json for his configuration files.
So if a file begin with { or [ it interprets it as json. The problem is that yaml sometimes
also starts with {

The solution is to use the default_flow_style=False option with the yaml.dump function to remove the parentesis.