In azure templates, the shared_image_gallery was mistakenly considered
an attribute while this is supposed to be a block.
This is due to a heuristic we use for deciding whether a JSON object is
to be translated to an attribute or a block that fell short as the
shared_image_gallery does not contain complex types.
This cannot be fixed trivially for the general case, so we add this
entity to the list of workarounds until we can implement something more
robust.
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