vm: as SSH key to connect to guest, use ed25519 type instead of rsa

By default, recent SSH (since v8.6) refuses to connect to not-so-old
server (e.g. the version on Debian 8) with RSA key.

slapos/recipe/vm.py

@@ -278,7 +278,8 @@ class InstallDebianRecipe(BaseRecipe):
     with building_directory(location):
       if vm_run:
         key = self.ssh_key
-        subprocess.check_call(('ssh-keygen', '-N', '', '-f', key), env=env)
+        subprocess.check_call(('ssh-keygen', '-N', '', '-t', 'ed25519',
+                               '-f', key), env=env)
         key += '.pub'
         with open(key) as f:
           os.remove(key)