backends/none: Use dedicated SSH keypair.

This should make the "none" backend a bit more useful as we only need to
provide a key _or_ passphrase on machine creation and from thereon we
only use the SSH keys within NixOps' state database.

If you have a large set of "none" machines and want to export/import
them, this should no longer require to move around ssh_configs or even
entries in your hosts file.

In addition, this fixes the situation where you need to build on the
target machines and the none backend wasn't able to provide a private
key for the to be generated nix.machines file.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Fix VM tests for hetzner and none backends.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
tests: Add Archive::Cpio to all the coordinators.

This is needed in order to build the initrd for the target machines and
we don't have access to external network resources within the VM test.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
backends/none: Correctly check for existence.

We now set up a vm_id, so on "nixops check" we can properly show whether
the target machine exists or not.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
backends/none: Provide a resource ID value.

The value is the vm_id we generated on create() and currently is more or
less just for consistency and to check whether the vm_id exists on
"nixops info".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
tests: Disable packet filtering on all VM tests.

This is only a source for more errors, confusion, more complexity and we
really don't want to test the packet filter here, because we already
have tests in NixOS directly.

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