Implement LoggingFile.writelines()

Existing code calls sys.stdout.writelines() because this method is
expected as part of the file object protocol.  When run in autotest
sys.stdout is actually a LoggingFile.  Implement writelines()
in LoggingFile to be more compliant with the File Object protocol.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>