Create the config backup archive in a safe way

Create the config backup archive in a safe way

Since the config backup archive contains sensitive information and is
written in world-readable locations (/var/lib by default), it should be
created in a safe way and with strict permissions.

This commit uses a temporary file to tackle two issues: the relaxed
permissions of the archive which respected the umask of the user running
`gnt-cluster upgrade' and a (possible) collision attack using a
pre-created file with the predictable backup filename.

Signed-off-by: Apollon Oikonomopoulos <apoikos@gmail.com>
Reviewed-by: Helga Velroyen <helgav@google.com>