Systems: Make publicKey in credential optional. Compute from privateKey

[scblack321]

We should be able to make providing a publicKey for a PKI credential optional. This will simplify registering credentials for a system.
Since our ssh library requires both a public and private key we will need to determine the public key from the private key.
It's not clear if the apache sshd library has a convenient way of doing this but it does look like standard java libraries can do this for RSA type key pairs.

This may be useful: https://stackoverflow.com/questions/8434428/get-public-key-from-private-in-java