Add skip_on_exit_code to SSHOperator

[dolfinus]

Now SSHOperator supports option skip_on_exit_code, just like BashOperator, PythonVirtualenvOperator and others.

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[josh-fell]

Suggested change

	if expected_exc is None:
	operator.execute({})
	else:
	with pytest.raises(expected_exc):
	operator.execute({})
	with pytest.raises(expected_exc):
	operator.execute({})

It's a nit really, but you don't need the branch here.

[dolfinus]

There are test cases with exit code 0 which does not raise exception. But passing None or empty tuple to pytest.raises is not supported:
ValueError: Expected an exception type or a tuple of exception types, but got None. Raising exceptions is already understood as failing the test, so you don't need any special code to say 'this should never raise an exception'.
ValueError: Expected an exception type or a tuple of exception types, but got (). Raising exceptions is already understood as failing the test, so you don't need any special code to say 'this should never raise an exception'.

[hussein-awala]

Nit