Support CentOS7 builds too
I'm planning to set up a build system that does continuous
delivery on top of both CentOS 7 and Fedora 22, using upstream
git and Fedora rawhide (or 22) spec files.
To help this work, tweak the Fedora spec to support CentOS 7 which is
Python 2.
Side note: This patch drops `BuildRequires: docker` because that
doesn't work. It does add a `BuildRequires: python-docker-py` which
is required.