docker: Install cffi and cryptography from system packages

Currently, builds are busted because they can't build wheels for these.

Change-Id: I964d5858bfb926890169d46cbc7ab4e76fc40250

docker/install_scripts/20_apk_install_py2.sh

@@ -4,4 +4,6 @@ set -e
 apk add --update \
   python \
   python-dev \
-  py-pip
+  py-pip \
+  py-cffi \
+  py-cryptography

docker/install_scripts/21_apk_install_py3.sh

@@ -4,7 +4,9 @@ set -e
 apk add --update \
   python3 \
   python3-dev \
-  py3-pip
+  py3-pip \
+  py3-cffi \
+  py3-cryptography
 
 if [ ! -e /usr/bin/pip ]; then ln -s pip3 /usr/bin/pip ; fi