make it work in CircleCI env

[yaroslavvb]

Currently failing because public keypair is only partially setup

('still waiting for thread make_task(1.integration_test)',) {}
('Timeout exceeded (60 sec) for /home/circleci/.ssh/id_rsa.pub',) {}
Exception are  [AssertionError('Private key /home/circleci/.ssh/id_rsa exists but public key /home/circleci/.ssh/id_rsa.pub not found, delete /home/circleci/.ssh/id_rsa and run again to regenerate pair',), AssertionError("Couldn't find /home/circleci/.ssh/id_rsa.pub",)]
Traceback (most recent call last):
  File "tests/integration_test.py", line 68, in <module>
    main()
  File "tests/integration_test.py", line 54, in main
    launcher()
  File "tests/integration_test.py", line 44, in launcher
    job = ncluster.make_job(**vars(args))
  File "/home/circleci/repo/venv/lib/python3.6/site-packages/ncluster/aws_backend.py", line 1278, in make_job
    raise exceptions[0]
  File "/home/circleci/repo/venv/lib/python3.6/site-packages/ncluster/aws_backend.py", line 1259, in make_task_fn
    **kwargs)
  File "/home/circleci/repo/venv/lib/python3.6/site-packages/ncluster/aws_backend.py", line 1026, in make_task
    util.setup_local_ssh_keys()
  File "/home/circleci/repo/venv/lib/python3.6/site-packages/ncluster/util.py", line 299, in setup_local_ssh_keys
    assert os.path.exists(ID_RSA_PUB), f"Private key {ID_RSA} exists but public key {ID_RSA_PUB} not found, delete {ID_RSA} and run again to regenerate pair"
AssertionError: Private key /home/circleci/.ssh/id_rsa exists but public key /home/circleci/.ssh/id_rsa.pub not found, delete /home/circleci/.ssh/id_rsa and run again to regenerate pair
Exiting