Merge lp://staging/~sinzui/juju-release-tools/build-centos into lp://staging/juju-release-tools

Build centos7 agents and clients.

This branch adds a centos command to crossbuild.py to create both agent and client tar files.

    ./juju-release-tools/crossbuild.py -v centos -b ~/Work/crossbuild ./juju-core_1.24-beta5.tar.gz

Which made
    agent: juju-1.24-beta5-centos7-amd64.tgz
    client: juju-1.24-beta5-centos7.tar.gz

We only support centos7. It is built as linux amd64 using golang-1.2.1 as recommended by cloudbase. We know the juju built on trusty amd64 will sort of work on other linuxes. The errors are caused by unregistered series and packaging differences. 1.24 registered centos7 and packaging rules for it.

The code changes are small because centos agent rules are like windows, and client rules are like OS X. I renamed two functions to make them generic, and take the series and os_names
    make_win_agent_tarball() => make_agent_tarball(series)
    make_osx_tarball() => make_client_tarball(os_name)

The build_centos function call for the build, then calls for both tar files. The build is native, so the binaries are installed directly into the gopath/bin dir instead of a cross-complied subdir.

I took a moment to use the one-true way to express an octal. I also extracted autopatch so that I could re-use it.