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 
Small grammar thing -- "has" should be "have".