We're being too strict - there is a third possibility, which is that a
user will have defined the OS_* environment variables and expect them to
pass through.
This PR adds the option to retry failed ssh executions, if the failure
is caused by ssh itself, not the remote command. This can be helpful if
there are transient network issues. Retries are only implemented in the
openssh connection plugin and are disabled by default. Retries are
enabled by setting ssh_connection > retries to an integer greater
than 0.
Running a long series of playbooks, or a short playbook against a large
cluster may result in transient ssh failures, some examples logged
[here](https://trello.com/c/1yh6csEQ/13-ssh-errors).
Ansible should be able to retry an ssh connection in order to survive
transient failures.
Ansible marks a host as failed the first time it fails to contact it.
