ssh_util: Allow to pass a command to invoke_shell.

This should make it way easier to use something like:

nixos ssh machine htop

We always used the logged command execution before which not only
clutters up stuff such as htop, but it also made no sense because the
commend is only to be used for single machines anyway.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
scripts/nixops: Properly quote args.args.

This uses the pipes.quote() function, that is available and documented
in Python 3.2 as shlex.quote(). Although undocumented in 2.x it already
exists and probably will only vanish in 3.x or later, and that's an
entirely different story.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
script/nixops: Stop parsing flags in ssh command.

This now makes it possible to use something like:

nixops ssh-for-each ls -l /

Without getting an error about "-l" being an unknown commandline
argument. But of course, arguments _before_ the command are parsed.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>