Generate server public key by provided private key

[svoitovich]

Currently to start rsynk one needs to pass both private and public parts of the ssh key. It's done so because apache sshd KeyPairProvider designed in a way it receives both.

Obviously we don't need a public part of a key if we have private.

Public key configuring should be removed from API. Public key must be read from the private.

To start take a look at RsynkBuilder.kt file.