In order to update the 'ganeti_pub_keys' and the
'authorized_keys' files of various nodes via SSH, we
introduce the tool 'ssh_update'. It works similar to the
tool 'prepare_node_join', which is also a tool invoked
via SSH on a remote note.

This patch includes some refactoring to reuse code from
the 'prepare_node_join' tool and provides unit tests as
well. Note that the actual invocation of the 'ssh_update'
tool will be done in later patches of this series.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>