Simple-aws-ec2

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**Bugfixes**

- fix a bug that ``EC2MetadataCache.get_xyz()`` methods always return ``None``.

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**Features and Improvements**

- add :class:`~simple_aws_ec2.ec2_metadata_cache.EC2MetadataCache`.

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**Features and Improvements**

- add a few ec2 metadata api methods for :meth:`~simple_aws_ec2.ec2.Ec2Instance`.

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**Features and Improvements**

- add :meth:`~simple_aws_ec2.ec2.Ec2Instance.terminate_instance` method.

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**Features and Improvements**

- add :meth:`~simple_aws_ec2.ec2.Ec2Instance.wait_for_status` waiter method.
- add :meth:`~simple_aws_ec2.ec2.Ec2Instance.wait_for_running` waiter method.
- add :meth:`~simple_aws_ec2.ec2.Ec2Instance.wait_for_stopped` waiter method.
- add :meth:`~simple_aws_ec2.ec2.Ec2Instance.wait_for_terminated` waiter method.

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**Bugfixes**

- Fix a bug that :meth:`~simple_aws_ec2.ec2.Image.os_type` returns ``None`` when it fails to guess, however, it should raise an exception.