Fix call to load_privatekey() when passphrase is None
Fix call to load_privatekey() when passphrase is None

The original code worked, but breaks with PyOpenSSL 0.15.1,
the version currently used by the gate.

Closes-Bug: #1448193
Change-Id: Iae44f08fa6442e3463e6b552955229f3fd36fbde

openstack/barbican,openstack/barbican,MCDong/barbican,MCDong/barbican,cloudkeep/barbican,cloudkeep/barbican,cneill/barbican,cneill/barbican