Make ssh-key-upload script more robust

It now works for both the case where we need to enter a password and the
password-less case.
Allow for specifying the SSH key that should be uploaded

Until now "SshKeysImporter" uploaded all default SSH keys to the system.
In some cases we want to upload a special key, which is not available to
the SSH agent, though. That is now possible by specifying the identity
file in the "import" call.
Merge pull request #4 from SUSE/review_140804_improve_ssh_key_upload

[Review] Request from 'aduffeck' @ 'SUSE/pennyworth/review_140804_improve_ssh_key_upload'