automatically create .ncluster directory

[yaroslavvb]

    image_name=args.image_name)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/ncluster/ncluster.py", line 83, in make_task
    install_script=install_script, **kwargs)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/ncluster/aws_backend.py", line 803, in make_task
    instance_type=instance_type)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/ncluster/aws_backend.py", line 99, in __init__
    self.ssh_client = u.ssh_to_task(self)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/ncluster/aws_util.py", line 378, in ssh_to_task
    pkey = paramiko.RSAKey.from_private_key_file(ssh_key_fn)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/paramiko/pkey.py", line 206, in from_private_key_file
    key = cls(filename=filename, password=password)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/paramiko/rsakey.py", line 55, in __init__
    self._from_private_key_file(filename, password)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/paramiko/rsakey.py", line 175, in _from_private_key_file
    data = self._read_private_key_file("RSA", filename, password)
  File "/Users/yaroslavvb/anaconda3/envs/main/lib/python3.6/site-packages/paramiko/pkey.py", line 278, in _read_private_key_file
    with open(filename, "r") as f:
FileNotFoundError: [Errno 2] No such file or directory: '/Users/yaroslavvb/.ncluster/ncluster-yaroslavvb-331439827203-us-east-1.pem'