logs directory for gc.log doesn't exist on first start

This was originally reported at https://stackoverflow.com/questions/47976248/gc-log-file-error-when-running-cassandra.

This is very minor problem related to timing of 'logs' directory creation - when Cassandra starts first time, this directory doesn't exist, and created when Cassandra starts to write system.log & debug.log files. But this directory is referenced in the -Xloggc command line parameter of JVM, causing following warning:

Java HotSpot(TM) 64-Bit Server VM warning: Cannot open file bin/../logs/gc.log due to No such file or directory

The fix is to check existence of this directory in the cassandra-env, and create it.