- remove "platform" key, see #1440. this implicitly sets schema to "basic". pending some other clean-up regarding json and mqtt queueing, other schema can be added down the line 
- updated ws module queue elem to capture callbacks list, allows to pass more than one callback (for example, when they are generated on the fly as lambdas, see ha wsPost usage)
- modified method to send ha config to use global ws queue, fix #1762 problem with empty topics and ensure json allocation is consistent.
- use existing defines to set mqtt payload options. amend #1085, #1188, #1883 to use the set payload value. drop HOMEASSISTANT_PAYLOAD... defines. 
- update MQTT_STATUS_ONLINE/OFFLINE and RELAY_MQTT_ON/OFF with runtime configuration
- filter payload strings so that the resulting yaml value is not interpreted as bool (python True, False)
- helper method for settings to streamline string values manipulation