Split x509.load_certificate() into PEM/DER functions
Splitting the load_certificate() function into two separate helps
us word the requirements for the input explicitly. It also makes
our backend similar to the one of python-cryptography so eventually
we can swap python-cryptography for IPA x509 module.
Reviewed-By: Fraser Tweedale <email@example.com>
Reviewed-By: Rob Crittenden <firstname.lastname@example.org>
Reviewed-By: Martin Basti <email@example.com>