add exec command

[ivotron]

It would be nice if one could execute an arbitrary command on a container (using docker-py's exec_create, exec_run and exec_inspect). Something like:

maestro exec <container-id> echo "hello world"

[mpetazzoni]

I'm torn on this. On the one hand, I can see the benefit of letting Maestro take care of figuring out where <container> is running for you, making it easier to execute a command inside that container amongst all your running containers.

On the other hand, the goal of Maestro is not to be a replacement command-line.

[ivotron]

I agree on the latter. Having to execute commands on containers might be an
indication that containers are not being used properly. This is kind of
equivalent to running sshd
https://jpetazzo.github.io/2014/06/23/docker-ssh-considered-evil/ in a
container.

having said this, there are times (e.g. in a dev environment) when you just
want to lunch a command for very ad-hoc reasons. As you mentioned, this
automates the task of figuring out the host IP/port and then executing
docker directly on the CLI.

The particular use case that I have in mind is to add support for docker in
teuthology https://github.com/ceph/teuthology (the Ceph integration
testing framework) which relies on being able to launch commands remotely.
The problem is that the tool tests multiple deployments (bare-metal and
VMs), relying on SSH to orchestrate tasks across the ceph cluster.