Updated man pages with new SPICE TLS options

man/gnt-cluster.rst:
* documented the --new-spice-certificate, --spice-certificate and
  --spice-ca-certificate options of renew-crypto.

man/gnt-instance.rst:
* documented the spice_use_tls KVM hypervisor option.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>