Removed the option os.O_EXCL for open in os_wr function

	 Removing the option os.O_EXCL, since its useful in many cases, like
	 appending on already created files. When you are using randomnames
	 (which is enabled by default) this won't be a problem, you can still
	 keep track of amount of data written. If you want to do some appending
	 on the existing files, disable random names, and rerun the same.

Signed-off-by: Vijaykumar Koppad <vijaykumar.koppad@gmail.com>