Support for Cloud Foundry Auth methos

[kamalakarp]

HashiCorp Vault has recently rolled out Cloud Foundry Auth method https://www.vaultproject.io/docs/auth/cf/. This is something all the cloud foundry users benefit out of it. I have a working code and example and Please let me know if you want to add to this.
I can do the PR and included as the additional feature.

[kamalakarp]

I wanted to add loginByCf method in Auth.java class