* Allow acl in mqtt
Acl in mqtt needs user name if 'use_identity_as_username' is not true. It also seems that login in mqtt don't accept an user with an empty password, then reenabled both
* Add new define for mosquitto
Keep compatibility with AWS_IOT
* Better approach to custom client certificate
Don't duplicate defined check. Let USE_MQTT_AWS_IOT automatically set USE_MQTT_CLIENT_CERT and use latter in all code
* with mosquitto user and password may be empty
When mosquitto 'use_identity_as_username' option is enabled password and user are not used
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