Rework the PKCS12-to-RSA conversion example
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.

juliankrispel/google-api-nodejs-client,jagandecapri/google-api-nodejs-client,stems/google-api-nodejs-client,danielrasmuson/google-api-nodejs-client,stems/google-api-nodejs-client,googleapis/google-api-nodejs-client,0111001101111010/google-api-nodejs-client,digideskio/google-api-nodejs-client,stems/google-api-nodejs-client,masterviana/google-api-nodejs-client,sqrrrl/google-api-nodejs-client,sqrrrl/google-api-nodejs-client,DIREKTSPEED-LTD/google-api-nodejs-client,googleapis/google-api-nodejs-client,sqrrrl/google-api-nodejs-client,tbetbetbe/google-api-nodejs-client,sqrrrl/google-api-nodejs-client,sqrrrl/google-api-nodejs-client,mcanthony/google-api-nodejs-client,benjamin-chang/google-api-nodejs-client,codeRuth/google-api-nodejs-client,googleapis/google-api-nodejs-client,PeterWangPo/google-api-nodejs-client,chitsuhein/google-api-nodejs-client,dstnation/google-api-nodejs-client,stems/google-api-nodejs-client,naskogithub/google-api-nodejs-client,stems/google-api-nodejs-client,thdtjsdn/google-api-nodejs-client