Integrate matrix-dynamic-dns with matrix-nginx-proxy without causing a dependency

We'd like the roles to be self-contained (as much as possible).

Thus, the `matrix-nginx-proxy` shouldn't reference any variables from
other roles. Instead, we rely on injection via
`group_vars/matrix_servers`.

Related to #681 (Github Pull Request)

group_vars/matrix_servers

@@ -851,6 +851,8 @@ matrix_ssl_architecture: "{{
 	}[matrix_architecture]
 }}"
 
+matrix_ssl_pre_obtaining_required_service_name: "{{ 'matrix-dynamic-dns' if matrix_dynamic_dns_enabled else '' }}
+
 ######################################################################
 #
 # /matrix-nginx-proxy