Implement replacing cluster certs and keys via “gnt-cluster renew-crypto”

Recent changes to “gnt-cluster verify” made it complain on expiring SSL
certificates. While it was possible to replace the SSL certificates and
other cluster secrets manually before, doing so was cumbersome. Cluster
certificates, keys and secrets can now be replaced easily.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>