The docker Ansible module was removed in Ansible 2.4. It was replaced by dedicated modules docker_container and docker_image.

[...]
2019-08-16 17:25:18,449 p=21450 u=mistral | TASK [Set internal tls variable] ***********************************************
2019-08-16 17:25:18,450 p=21450 u=mistral | Friday 16 August 2019 17:25:18 +0200 (0:00:00.498) 0:04:25.926 *********
2019-08-16 17:25:18,631 p=21450 u=mistral | ok: [ctr2-xx-yyy-1] => {"ansible_facts": {"internal_tls_enabled": true}, "changed": false}
2019-08-16 17:25:19,085 p=21450 u=mistral | TASK [remove TLS proxy if configured and running] ******************************
2019-08-16 17:25:19,086 p=21450 u=mistral | Friday 16 August 2019 17:25:19 +0200 (0:00:00.636) 0:04:26.562 *********
2019-08-16 17:25:19,760 p=21450 u=mistral | fatal: [ctr2-xx-yyy-1]: FAILED! => {"changed": false, "msg": "This module has been removed. The module documentation may contain hints for porting"}
2019-08-16 17:25:19,766 p=21450 u=mistral | PLAY RECAP *********************************************************************

PLAY RECAP *********************************************************************
ctr2-xx-yyy-1 : ok=59 changed=26 unreachable=0 failed=1

Friday 16 August 2019 17:25:19 +0200 (0:00:00.682) 0:04:27.245 *********
===============================================================================

Ansible failed, check log at /var/lib/mistral/1e6cc465-bb47-471f-95bc-fa37318dd45f/ansible.log.