roles/synapse: Add cert role dependency

The *cert* role must be defined as a role dependency now, so that the
role can define a handler to "listen" for the "certificate changed"
event.  This change happened on *master*, before the *matrix* branch was
merged.

synapse.yml

@@ -2,13 +2,6 @@
   vars_files:
   - vault/synapse/{{ matrix_server_name }}
   roles:
-  - role: cert
-    cert_src: '{{ matrix_tls_cert }}'
-    cert_dest: /etc/pki/tls/certs/{{ matrix_tls_cert|basename }}
-    cert_key_src: '{{ matrix_tls_key }}'
-    cert_key_dest: /etc/pki/tls/private/{{ matrix_tls_key|basename }}
-    when: matrix_tls_cert is defined
-    tags: cert
   - apache
   - synapse
   tasks: