Wrap all asset read operations.

First step in moving assets out of the contentstore (optionally).
https://openedx.atlassian.net/browse/PLAT-77
You only use `asset` in this line, and it might be clearer to just pass the arguments directly, rather than using a `dict` and `**`. Same comment goes for `asset_filename`.

Although, if you expect to be asserting on the `asset` and `asset_filename` later, than this is fine.