Bugfix: Failed executing command: 'free -s 1 -c 10'

Bugfix: Failed executing command: 'free -s 1 -c 10'

JIRA: YARDSTICK-585

In CI when run tc070, there is a error: Failed executing command: 'free -s 1 -c 10'
Here it is the log:
Traceback (most recent call last):
   File
   "/usr/local/lib/python2.7/dist-packages/yardstick/benchmark/runners/duration.py",
   line 69, in _worker_process
   method(data)
   File
   "/usr/local/lib/python2.7/dist-packages/yardstick/benchmark/scenarios/compute/memload.py",
   line 126, in run
   result.update(self._get_mem_usage())
   File
   "/usr/local/lib/python2.7/dist-packages/yardstick/benchmark/scenarios/compute/memload.py",
   line 116, in _get_mem_usage
   result = self._execute_command(cmd)
   File
   "/usr/local/lib/python2.7/dist-packages/yardstick/benchmark/scenarios/compute/memload.py",
   line 70, in _execute_command
   cmd, stderr)
RuntimeError: ('Failed executing command: ', 'free -s 1 -c 10',u"free: seconds argument `1' failed\n")
And it is a bug of free. the -c option should in front of -s, so change the position will solve
this problem.

Also it has another bug:
'KeyError', there no 'cached' keyword, so I change it to 'buff/cache'.

Change-Id: I0ca16e8d8cc11c6a3b2f364cadbdb3ea367eee53
Signed-off-by: chenjiankun <chenjiankun1@huawei.com>