add option to use scp instead of rsync in ManagedFile for syncing files to minimal targets

[flxzt]

Some embedded boards only have a minimal system only with core utilities available. So rsync cannot be used to sync files with them. But if there is ssh access (openssh, dropbear, ..) it is more likely that they have sftp implemented.

I am proposing to replace the rsync call (here) with scp or at least give the option to choose.

It can work in combination with providing ssh base args, as long as they all are converted to the config syntax with -o opt=val, e.g. -x becomes -o ForwardX11=no and so on.

an initial implementation can be found here: https://github.com/flxzt/labgrid/tree/ssh-put-file-scp