Unfortunately, the same /etc/init/ambari-agent.conf file was used for all flavours of Linux.
And in CentOS/RHEL6, the line 'kill signal SIGKILL' from this file is not compatible with the older version of upstart used in CentOS/RHEL6.
Ansible service module would always prefer to use upstart instead of SysVinit to start services, hence the error, as the upstart stop/start ambari-agent does not work in CentOS/RHEL6.
This is fixed in Ambari 188.8.131.52 so I suggest you use Ambari 184.108.40.206.
If you can't, then you'll need to use the following Ansible task before your service task:
- name: Fix for upstart script in RHEL6
when: ansible_os_family == "RedHat" and ansible_distribution_major_version == "6"