Create base_attachment_object_storage to extract common code to store implementations

attachment_swift/README.rst

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+Attachments on Swift storage
+============================
+
+This addon allows to store the attachments (documents and assets) on 
+OpenStack Object Storage (Swift)
+
+Configuration
+-------------
+
+Activate Swift storage:
+
+* Create or set the system parameter with the key ``ir_attachment.location``
+  and the value in the form ``swift``.
+
+Configure accesses with environment variables:
+
+* ``SWIFT_HOST``
+* ``SWIFT_ACCOUNT``
+* ``SWIFT_PASSWORD``
+* ``SWIFT_WRITE_CONTAINER``
+
+Read-only mode:
+
+The container name and the key are stored in the attachment. So if you change the
+``SWIFT_WRITE_CONTAINER`` or the ``ir_attachment.location``, the existing attachments
+will still be read on their former container. But as soon as they are written over
+or new attachments are created, they will be created on the new container or on
+the other location (db or filesystem). This is a convenient way to be able to
+read the production attachments on a replication (since you have the
+credentials) without any risk to alter the production data.
+
+This addon must be added in the server wide addons with (``--load`` option):
+
+``--load=web,web_kanban,attachment_swift``