Benchmarks: Fix bug - Fix missing key error in disk performance benchmark (#188)

**Description**
This commit fixes error of missing key 'percentile' in parsing FIO result.

superbench/benchmarks/micro_benchmarks/disk_performance.py

@@ -204,7 +204,8 @@ def _process_raw_result(self, cmd_idx, raw_output):
                 self._result.add_result('%s_iops' % io_type_prefix, float(iops))
 
                 for lat_unit in lat_units:
-                    if lat_unit in fio_output['jobs'][0][io_type]:
+                    if lat_unit in fio_output['jobs'][0][io_type] and \
+                       'percentile' in fio_output['jobs'][0][io_type][lat_unit]:
                         lat_unit_prefix = '%s_%s' % (io_type_prefix, lat_unit)
                         for lat_percentile in ['95.000000', '99.000000', '99.900000']:
                             lat = fio_output['jobs'][0][io_type][lat_unit]['percentile'][lat_percentile]