storage_service: Make decommission log info instead of debug level
storage_service: Make decommission log info instead of debug level

The log is just a few lines. It is very useful to tell which step fails
in case of error when we do decommission.

kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla