X509 uses ASN1 GENERALIZEDTIME or UTCTIME to store certificate validity.
pyOpenSSL 0.7 and above allow us to retrieve both “notBefore” and
“notAfter” as strings. Parsing them turned out to be a challenge since
they can be in a variety of formats (YYYYMMDDhhmmssZ, YYYYMMDDhhmmss+hhmm
or YYYMMDDhhmmss-hhmm).

This will be used to verify the validity of cluster certificates in
LUVerifyCluster.

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