configstore: set base path for datamanager

Don't put datamanager base dirs inside the root of the ost but use a base path.

Let's do it now before releasing since this is a breaking change that requires
moving the ost data to the new path

internal/services/configstore/configstore.go

@@ -72,6 +72,7 @@ func NewConfigstore(ctx context.Context, c *config.Configstore) (*Configstore, e
 	}
 
 	dmConf := &datamanager.DataManagerConfig{
+		BasePath: "configdata",
 		E:        e,
 		OST:      ost,
 		DataTypes: []string{