add-machine for centos aborts in cloud-init at firewalld

[John George]

'juju add-machine --series centos7', will abort in cloud init, when it calls 'systemctl stop firewalld'.
Machine status remains in "pending" indefinitely.

This bug can easily be reproduced by bootstrapping with a MAAS provider, that has centos images installed, followed by 'juju add-machine --series centos7'. The machine status will remain in pending but finding the IP of the machine and logging in will allow access to /var/log/cloud-init-output.log, which shows the error.

Please see attached logs.

[Cheryl Jennings (cherylj)]

This appears to be a dup of 1492066, which was fixed in 1.25, but not 1.24.