Switch to python-cffi for rpm based distros

Note that latest version of python-cffi is provided
from CentOS base and is python-cffi instead of python2-cffi.

According to CentOS/RDO packaging rules, all python2-XXX
packages has a provides on python-XXX, so when installing
it, python-XXX should always be used as it will install the
right package.

Change-Id: Ie30901c64993a6031278812e5072046a50371b3d
Closes-Bug: #1728915

docker/openstack-base/Dockerfile.j2

@@ -40,7 +40,6 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
             'MySQL-python',
             'pyOpenSSL',
             'pyparsing',
-            'python2-cffi',
             'python-crypto',
             'python2-cryptography',
             'python2-debtcollector',
@@ -76,6 +75,7 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
             'python-beaker',
             'python-cachetools',
             'python-ceilometerclient',
+            'python-cffi',
             'python-cliff',
             'python-cloudkittyclient',
             'python-cmd2',