Verify cluster certificates in LUVerifyCluster

When using pyOpenSSL 0.7 or above, LUClusterVerify will start to show a
warning 30 days before a certificate expires. 7 days before the
certificate expires, the warning becomes an error. Once expired,
LUVerifyCluster will always report an error. The latter is also supported
with pyOpenSSL 0.6.

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