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.