Add pyca/cryptography to global requirements

barbican would like to convert some of its internal encryption
to using the cryptography project from PyCA. Specifically our
dev plugin would benefit from a switch to cryptography as it
allows us to use AEAD modes. We will also need this in the
future as cryptography adds X509 and CMS support.

The project supports Python 2.6-3.4 as well as PyPy and is being
packaged for FreeBSD, Debian, and Ubuntu.

Additionally, this is already a second-level requirement when
using PyOpenSSL.

Change-Id: Ic0d508faef9de347bf7cb2436a433096779cf8ac
Merge "Add pyca/cryptography to global requirements"