Add the storage id to the mount config

First step into improving the config is making sure we have a stable id to work with.

cc @DeepDiver1975 @PVince81 
Looks like we're writing the config file for every setup/file access ?
I could imagine cases with concurrency issue when multiple HTTP calls are calling setupFS() which will call initMountPointsHook() and write this config at the same time... who knows in what state the file will be ?

Maybe it would be better to first check whether there's an id in the config. If none was found, then write it.
But don't write it every time.