Implemented review comments

I've implemented following adam's review comments:

1.command is knife certgen, not knife winrm certgen. So the source files need to be moved under winrm or named winrm_certgen, etc.
2.Too much screen output from certgen command -- we should have output similar to ssh-keygen.
3.It ignored the path I specified as an argument to certgen, and always generates winrmcert* files in the current directory.

^@muktaa 
Apologies for confusion on the "consistency" statements -- the knife certgen command was fine since these certs could be used for something other than winrm (in theory). The key thing is for the option names to be consistent, so options specific to winrm stuff should start with --winrm on knife winrm cert install, etc. Here are some guidelines:

1. By default, assume nothing is winrm specific, so options should not start with --winrm.
2. If the cert install and listener create commands have options that are truly only in the winrm domain, then those options should start with --winrm.
3. If the cert generate command is using something that corresponds to one of the aforementioned --winrm options, it should also have --winrm.

I'll send a list for each command.