Update api and mqtt timeout configs to use substitution

common/config/api.yaml

@@ -2,6 +2,6 @@
 ## https://esphome.io/components/api.html
 api:
   # Set to 0s to disable switch reboot if nothing connects to the API
-  reboot_timeout: 15min
+  reboot_timeout: ${api_reboot_timeout}
   encryption:
     key: ${encryption_key}

common/config/mqtt.yaml

@@ -2,7 +2,7 @@
 ## https://esphome.io/components/mqtt.html
 mqtt:
   # Set to 0s to disable switch reboot if no MQTT connection exists
-  reboot_timeout: 15min
+  reboot_timeout: ${mqtt_reboot_timeout}
   broker: ${mqtt_broker}
   username: ${mqtt_username}
   password: ${mqtt_password}