Acore-server-metadata

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**Features and Improvements**

- Now the ``acore_server_metadata.api.Server.get_server()`` method will always return a ``Server`` object. If the ec2 or rds doesn't not exists, then the ``ec2_inst`` or ``rds_inst`` attribure of the ``Server`` object will be ``None``. This behavior was returning ``None`` before.
- Similarly the ``acore_server_metadata.api.Server.batch_get_server()`` method will always return a ``Server`` object for specific id.

**Minor Improvements**

- add many unit test to cover the server operations API.

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**Bugfixes**

- fix a bug when "check=True", we didn't use the object representing the latest ec2 or rds metadata.

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**Minor Improvements**

- ``acore_server_metadata.api.Server.run_rds`` now also copy ``tech:master_password_digest`` tag from snapshot to RDS instance.

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**Minor Improvements**

- ``acore_server_metadata.api.Server.run_ec2`` and ``acore_server_metadata.api.Server.run_rds`` now receive additional kwargs. The ``allocated_storage`` is no longer mandatory.

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**Minor Improvements**

- ``acore_server_metadata.api.Server.associate_eip_address`` now returns API response or None
- ``acore_server_metadata.api.Server.update_db_master_password`` now returns API response or None
- ``acore_server_metadata.api.Server.cleanup_db_snapshot`` now returns API response or None
- add ``acore_server_metadata.api.Server.create_ec2``, it is a alias of ``run_ec2``
- add ``acore_server_metadata.api.Server.create_rds``, it is a alias of ``run_rds``

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**Features and Improvements**

- add the following public API:
- ``acore_server_metadata.api.Server.start_ec2``
- ``acore_server_metadata.api.Server.start_rds``
- ``acore_server_metadata.api.Server.stop_ec2``
- ``acore_server_metadata.api.Server.stop_rds``
- ``acore_server_metadata.api.Server.delete_ec2``
- ``acore_server_metadata.api.Server.delete_rds``