{% set vars={'hosts': ''} %}

{% for h in groups['all'] %}
{% if h != 'bastion' %}
{% if vars.update({'hosts': vars['hosts'] + ' ' + hostvars[h]['ansible_ssh_host']}) %}{% endif %}
{% endif %}
{% endfor %}

Host {{ bastion_ip }}
  Hostname {{ bastion_ip }}
  StrictHostKeyChecking no
  ControlMaster auto
  ControlPath ~/.ssh/ansible-%r@%h:%p
  ControlPersist 5m

Host {{ vars['hosts'] }}
  ProxyCommand ssh -W %h:%p {{ real_user }}@{{ bastion_ip }}
  StrictHostKeyChecking no