Add matrix_synapse_cas_config_enabled variable to make cas_confg non-null

This works around an issue with Matrix Authentication Service's `syn2mas` sub-command (at version v0.16.0),
which chokes with an error:

> Error: Failed to load Synapse configuration
> Caused by:
> invalid type: found unit, expected struct EnableableSection for key "default.cas_config" in homeserver.yaml YAML file

This issue is likely to be fixed in MAS v0.16.1 or v0.17.0.

roles/custom/matrix-synapse/defaults/main.yml

@@ -725,6 +725,9 @@ matrix_synapse_oidc_providers: []
 # Note: internally, this uses the `--mount` flag for mounting the specified volumes.
 matrix_synapse_container_additional_volumes: []
 
+# Controls whether cas_config is enabled
+matrix_synapse_cas_config_enabled: false
+
 # A list of additional loggers to register in synapse.log.config.
 # This list gets populated dynamically based on Synapse extensions that have been enabled.
 # Contains definition objects like this: `{"name": "..", "level": "DEBUG"}

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

@@ -2136,7 +2136,7 @@ cas_config:
   # Uncomment the following to enable authorization against a CAS server.
   # Defaults to false.
   #
-  #enabled: true
+  enabled: {{ matrix_synapse_cas_config_enabled | to_json }}
 
   # The URL of the CAS authorization endpoint.
   #