Improves performance of media store operations.

Silvio announced that he's no longer maintaining his images,
so we're jumping to AVENTER-UG's fork.

The non-working script is supposed to be fixed
by https://github.com/matrix-org/synapse/pull/2375

To have it work, we'd need an updated Docker image
of `silviof/matrix-riot-docker:latest`, which is not yet available
at the time of this commit.

Still, the previous patched synapse_port_db didn't work well either,
so it's not like we're regressing much by getting rid of it.

By using role defauts, we can have inventory variables
which overide the defaults.