Rework the PKCS12-to-RSA conversion example

Added the `-nodes` switch to disable passphrase for the pem certificate and also merged the commands together to make the conversion a single-step process. Also, since we are converting the file to *key.pem*, I updated the **JWT** constructor's certificate path to be *key.pem* instead of *private_key.pem* .

See discussion in #154.