[pve-devel] Add ssh command helpers and use them when migrating

[Thomas Lamprecht]

The indend of this is to replace the various places where we assemble a ssh
command manually with a helper which does that for us and uses common
options, like BatchMode, root as default login name.
This makes it also easier to make possible wanted changes to the way we
use SSH in the future, e.g. switch to another default user or change to an own daemon
(just to name a few possible examples). We would have to change just the
helper.

I added the HostKeyAlias to the standard options, as it allows to connect to
a known host over different IPs, this makes switching the cluster network or
using a dedicated migration network easier.