Explicitly serialize matrix_synapse_app_service_config_files

Attempt to fix #192 (Github Issue), potential regression since
70487061f4.

Serializing as JSON/YAML explicitly is much better than relying on
magic (well, Python serialization being valid YAML..).
It seems like Python may prefix strings with `u` sometimes (Python 3?),
which causes Python serialization to not be compatible with YAML.

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -893,7 +893,7 @@ report_stats: {{ matrix_synapse_report_stats|to_json }}
 
 # A list of application service config files to use
 #
-app_service_config_files: {{ matrix_synapse_app_service_config_files }}
+app_service_config_files: {{ matrix_synapse_app_service_config_files|to_json }}
 
 # Uncomment to enable tracking of application service IP addresses. Implicitly
 # enables MAU tracking for application service users.