Untrusted playbook task execution on executors

[Monty Taylor]

Dear Zuul operators,

The change to allow the use of add_host in untrusted playbooks, which 
was included in Zuul version 3.3.0, inadvertently opened the door for an 
user to abuse the ssh_executable parameter to cause a command to be run 
on the executor. We do not believe this would have given jobs access to 
the executor host as every job is sandboxed within a bubblewrap 
container with minimal access to the filesystem and other resources.

Further information can be found in StoryBoard: 
https://storyboard.openstack.org/#!/story/2004405

This is fixed by https://review.openstack.org/620635 which has been 
included in Zuul version 3.3.1. It is strongly advised to update to 
3.3.1 or latest master and restart any running zuul-executor processes.

Sorry for any inconveniences.

Thanks!
Monty