[Flink-1142] Log information about IOManager temp directories.

Log the percentage of available space on the disk (total, free, percentage)
Remove those from the list with less than the defined threshold (with a WARN-message)
I think the configuration value is not important enough to put it into the default configuration file (I think its a good idea not to overwhelm users with a huge configuration file)
I really like that you added the configuration value to the documentation.