Just to expand on tillda’s answer, that config can be placed in an ansible.cfg file alongside your playbook. e.g.:
ansible.cfg
[defaults]
transport = ssh
[ssh_connection]
ssh_args = -o ForwardAgent=yes
I’d say it’s better to do that than setting as an env variable, as placing it in a conf file is both more declarative and also will minimise the steps needed for other people you may be working with to going with a project.
Conf docs:
http://docs.ansible.com/intro_configuration.html#the-ansible-configuration-file
Example config file:
https://raw.github.com/ansible/ansible/devel/examples/ansible.cfg