MachineState: Use single SSH connection for keys.

It just doesn't make sense to initiate a separate connection for every
single key, so let's do it in one shot. In the long term this probably
should be consolidated even more by caching/pooling SSH connections.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
ssh_util: Don't use keys from agent or ~/.ssh.

We can still introduce the use of keys from ~/.ssh or the agent later,
but for now this behaviour is quite annoying (especially when debugging
connection issues), so let's turn it off by default.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
paramiko: Add patch for ECDSA private keys.

This overrides the paramiko derivation to include a patch that allowsa
to use ECDSA private keys, which is the default in the current version
of NixOps.

Previously I've added the patch in NixOS/nixpkgs@58fdf34, but as it is
largely NixOps specific, I revented it und thus added it here directly.

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