Rework SSHExec

[missytake]

SSHExec is an object in cmdeploy.py, which we can use to get the result of bash commands on the relay. Since it was last reworked in #396, our needs have evolved. Obstacles of the current implementation:

• it always invariably connects to mail_domain, the relay host
• it is initialized in cmdeploy.py's main() function, so the functions which come afterwards can hardly modify it
• it doesn't offer to simply pass a bash command, you need to pass a function to it which calls a remote.rshell method
• it offers two different callback functions for console output, but no option to suppress it completely
• the various functions which execute the bash command on the target system are spread out over many different files: cmdeploy.py, dns.py, sshexec.py, remote/rdns.py, and remote/rshell.py

Situations in which we need it to work differently:

• in WIP: cmdeploy command to deploy a reverse proxy #628, we want to run commands on a reverse-proxy
• in Add installation using Docker Compose and many other improvements for implementation in Docker. #614, we want to run commands locally
• in Make cmdeploy dns work on localhost/in docker containers Keonik1/relay#3, we want to run DNS queries locally instead of on the relay
• in docker-dns...docker-tests, we want to run commands in a docker container
• in cmdeploy: print echobot link at the end of cmdeploy run #620 we need to suppress the "[ssh] login to..." output

The current implementation has a certain elegance to it, to make it easier to test; I wonder whether it can be enhanced to fit all these needs. At the same time it would be nice to make it simpler and more maintainable (it took me a few weeks to understand the current obstacles well enough to write down this issue).