simplify the code and make sure the file is open for write
simplify the code and make sure the file is open for write

previously the file was open for append but didn't have the o_wronly flag. this
caused errors when lager was writing to the file

[#146622173]

Signed-off-by: Swetha Repakula <a5734a87406f5f357b62e206d655a02a4f6ee691@us.ibm.com>

cloudfoundry-incubator/benchmark-bbs