This is the equivalent of b1b4ba501f for the matrix-dendrite role.

This brings matrix-dendrite.service in line with all the other services.

We're trying to move away from implementation-specific variables,
hoping for a clean (implementation-neutral) examples/vars.yml file.

Doing this seems more reasonable and simpler.

Also get rid of `--tags=update-user-password` in the
`matrix-dendrite` role, as what we had doesn't work.

We may be able to do it with some Ansible helper or something else.
For now, we'll omit this feature.

We also need to fix `--tags=register-user`. This tag is currently
defined in both the `matrix-synapse` and `matrix-dendrite` roles.

All the `matrix_s3_*` stuff happens in the `matrix-synapse` role.

If we are to have such S3 support for Dendrite, we should probably
extract it out of the `matrix-synapse` role (into a `matrix-s3` role or
`matrix-goofys`, etc.) and wire `matrix-dendrite` accordingly.

This may or may not be done in the future though. For now, I'm
cleaning things up in the `matrix-dendrite` role.

Since 025a5ab5e5f6bdb3, we do a similar check in matrix-base.

This brings dendrite.yaml up to date and cleans things up a bit.

We refrain from logging to files for all components,
because we rely on systemd-journald anyway.