Demandshaper service should follow the convention of symlinking service file

[borpin]

https://github.com/emoncms/demandshaper/blob/e23c97ea8198e64df0c716c7126d048d01f6ea01/install.sh#L19

Instead of copying the file, demandshaper should follow the convention of symlinking the service file from the installed folder.

The EXEC path then does not need to be set.