Refactor success and createUser into SshUtil

To permit other forms of authentication to be performed,
having the generic set-up of the user and verification in a
common class allows this implementation to be called via
alternative authentication methods.

These changes have been migrated from DatabasePubKeyAuth into
SshUtil, with the only changes being from the required signatures
and a modification which doesn't require a key for the authentication
or success stage.

Change-Id: I59835e772d1f467ce5a49e8583064368cc3276b8
2 files changed