Prefer --mount instead of -v for mounting volumes

This doesn't replace all usage of `-v`, but it's a start.

People sometimes troubleshoot by deleting files (especially bridge
config files). Restarting Synapse with a missing registration.yaml file
for a given bridge, causes the `-v
/something/registration.yaml:/something/registration.yaml:ro` option
to force-create `/something/registration.yaml` as a directory.

When a path that's provided to the `-v` option is missing, Docker
auto-creates that path as a directory.
This causes more breakage and confusion later on.

We'd rather fail, instead of magically creating directories.
Using `--mount`, instead of `-v` is the solution to this.

From Docker's documentation:

> When you use --mount with type=bind, the host-path must refer to an existing path on the host.
> The path will not be created for you and the service will fail with an error if the path does not exist.

roles/matrix-synapse/tasks/ext/ldap-auth/setup.yml

@@ -1,8 +1,6 @@
 - set_fact:
     matrix_synapse_password_providers_enabled: true
-  when: matrix_synapse_ext_password_provider_ldap_enabled|bool
 
-- set_fact:
     matrix_synapse_additional_loggers: >
       {{ matrix_synapse_additional_loggers }}
       +