Validator Reassociation of signing key

Node enhancement proposal to provide node operators an ability to "transfer" their validator from one public key to another.

Validator transfer could be used to securely transfer ownership of a validator without exchanging secret keys.
Another use case would be migrating to a different machine without any keys ever leaving the original host.
Please find the detailed proposal in the CEP request [here](https://github.com/casper-network/ceps/pull/91/files)