tests: use sequence form of Popen constructor

This eliminates the need to quote the command passed to the remote host
thus making it easier to avoid shell escaping problems, and removes any
possible security risks of using shell=True.

Signed-off-by: Tim Serong <tserong@suse.com>
tests: ssh to minions as user specified in info.yaml

This removes the hard coded `ssh ubuntu@{target}`, which makes execution
on other distros problematic.

Signed-off-by: Tim Serong <tserong@suse.com>