Make runtime cgroups optional

This allows folks to develop other cgroup managers outside of the
runtimes, which is a prerequisite for cgroup removal [1].  I feel like
there's a consensus on this issue, so I filed a pull-request [2], but
it was punted back to the list [3].

[1]: Message-ID: <CAD2oYtO1RMCcUp52w-xXemzDTs+J6t4hS5Mm4mX+uBnVONGDfA@mail.gmail.com>
     Subject: removal of cgroups from the OCI Linux spec
[2]: https://github.com/opencontainers/specs/pull/237
[3]: https://github.com/opencontainers/specs/pull/237#issuecomment-152294240

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