Implementation of TLS-protected SPICE connections

Added support for TLS-protected SPICE connections:

client/gnt_cluster.py, cli.py:
* added three new parameters to renew-crypto (--new-spice-certificate,
  --spice-certificate, --spice-ca-certificate) and their validation.

utils/x509.py:
* changed GenerateSelfSignedSslCert so that now also returns the
  generated key and certificate;
* added missing return value in the docstring of
  GenerateSelfSignedX509Cert.

lib/bootstrap.py:
* changed the signatures of the relevant functions and implemented
  certificates generation/writing.

tools/cfupgrade:
* changed GenerateClusterCrypto invocation to reflect the new signature;
* added SPICE certificate names.

lib/errors.py:
* added the X509CertError class.

lib/hypervisor/hv_kvm.py:
* silenced pylint warning R0915

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