Execute commands on running workers

[tilmankamp]

Low hanging fruit: Allow similar experience as with lxc exec by

• streaming stdin/stdout/stderr through web-sockets to the Snakepit service and
• forwarding them from there to LXD web-sockets (of exec command/endpoint) of the target worker.

This would also allow users to log into running workers:

$ pit exec 1234:0 -- bash
root@sp-n0-1234-0:~# ls /
bin   code  dev  home  lib64  mnt  proc  run   snap  sys  usr
boot  data  etc  lib   media  opt  root  sbin  srv   tmp  var
root@sp-n0-1234-0:~# exit
$