Improving SSH configuration defaults (#4055)

 * `look_for_keys` is now set to `True` by default, which is also
   paramiko's default
 * `allow_agent` is now set to `True` by default (on the shell, if you
   have an SSH agent, you expect that it is automatically used by ssh)
 * If an empty string is passed as the key_filename, this is not passed
   to paramiko, so it will search for it rather than complaining that
   file `''` does not exist.

aiida/transports/plugins/ssh.py

@@ -254,15 +254,15 @@ def _get_allow_agent_suggestion_string(cls, computer):
         Return a suggestion for the specific field.
         """
         config = parse_sshconfig(computer.hostname)
-        return convert_to_bool(str(config.get('allow_agent', 'no')))
+        return convert_to_bool(str(config.get('allow_agent', 'yes')))
 
     @classmethod
     def _get_look_for_keys_suggestion_string(cls, computer):
         """
         Return a suggestion for the specific field.
         """
         config = parse_sshconfig(computer.hostname)
-        return convert_to_bool(str(config.get('look_for_keys', 'no')))
+        return convert_to_bool(str(config.get('look_for_keys', 'yes')))
 
     @classmethod
     def _get_proxy_command_suggestion_string(cls, computer):
@@ -405,6 +405,8 @@ def open(self):
             raise InvalidOperation('Cannot open the transport twice')
         # Open a SSHClient
         connection_arguments = self._connect_args
+        if 'key_filename' in connection_arguments and not connection_arguments['key_filename']:
+            connection_arguments.pop('key_filename')
         proxystring = connection_arguments.pop('proxy_command', None)
         if proxystring:
             self._proxy = _DetachedProxyCommand(proxystring)