Fix error "AttributeError: 'module' object has no attribute 'Cryptography_HAS_SSL_ST'" when installing keystone on Ubuntu 16.04

I followed the keystone's official docs to install it but got this error on Ubuntu 16.04:

AttributeError: 'module' object has no attribute 'Cryptography_HAS_SSL_ST'

Then I added this below ppa and I can install keystone successfully:

sudo add-apt-repository ppa:ubuntu-cloud-archive/queens-staging
sudo apt-get update
sudo apt install keystone apache2 libapache2-mod-wsgi