Remove or double-down on Paramiko dependency for SSH

[johnsca]

The manual provisioning code uses the Paramiko library to invoke commands on the new machines via SSH, but #393 shows that it may be simpler to simply use async subprocesses to the command-line SSH client. I think the original rationale was to enable support on systems that didn't have an ssh command-line client. If that's still a concern, then we should convert the model code to use Paramiko instead, but if it's not, it would be great to be able to remove that dependency.

[johnsca]

@davigar15 I believe you can shed some light on whether we need to be concerned about the potential lack of an ssh CLI client?

[AdamIsrael]

@johnsca The only reason for Paramiko, that I can recall, was because that's what I was familiar with at the time. I remember that we talked briefly about using asyncssh at one point.

I think it's safe to assume -- or require -- the machine running the provisioner to have an ssh client installed, though.

[juanmanuel-tirado]

Close. Follow comments on #399