Replace the ENV variable with a config file

So it's safer to configure, as we don't need to pass env variables
to a docker run service. Also it's easier to configure without
needing to pass a whole lot of env vars.

requirements.txt

@@ -1,2 +1,3 @@
-paho-mqtt=1.5.0
-solaredge_modbus=0.2.0
+paho-mqtt>=1.5.0
+solaredge_modbus>=0.2.0
+pyyaml>=5.3