Fix the Inconsistency in cf files command

Before Fix: If cf user does
'cf files my-app' or 'cf files my-app path'
then its success but path is niehter optional not required

After Fix:If cf user does
'cf files my-app path'
then it will succ  but 'cf files my-app' will fail
so path is now reqired parameter

http://rnd-github.huawei.com/paas/cli/issues/12#issuecomment-5478