Fix the Inconsistency in cf files command

[SrinivasChilveri]

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

[liuhewei]

@SrinivasChilveri As we discussed, this PR and issue should be kept open and following cf-cli's plan on V7.0.0.

[SrinivasChilveri]

@liuhewei I have implemented PATH as required parameter.we have not received any info from pivotal to implement as required or optional parameter, so can i do one more PR with PATH as optional parameter? So we will be ready with two options what ever they want we can do PR for that. whats your suggestion.