Removing required options to be able use config-file instead

Improve by checking if i.e Chef:Config[:knife][:image] already exists
from config-file.
adding options: identity-file, bootstrap and force_owerwrite

For use with solo option --no-bootstrap now works.
force_owerwrite might not be useful except for testing…
identity-file add given public key to authorized keys on server for
passwordless  login.