Make --tags=register-user work for both Dendrite/Synapse

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.

roles/matrix-base/defaults/main.yml

@@ -228,6 +228,7 @@ run_synapse_register_user: true
 run_synapse_update_user_password: true
 run_synapse_import_media_store: true
 run_synapse_rust_synapse_compress_state: true
+run_dendrite_register_user: true
 run_setup: true
 run_self_check: true
 run_start: true