Fix wrong retain value when setting in mqtt connection string

[cserem]

rtl433 expects numbers instead of true/false for retain flag. Don't know if this is the proper way to fix it, but it seems to work for me.

[deviantintegral]

Nice, thank you! This matches the docs at https://github.com/merbanan/rtl_433/blob/master/conf/rtl_433.example.conf#L189.