support linux.cgroupsPath and linux.resources (resource constraints)

[jodh-intel]

The OCI config file, config.json, specifies resource constraints in the form of linux.resources JSON objects which map closely to cgroups.

We need to find a way to implement such constraints.

See:

• https://github.com/opencontainers/runtime-spec/blob/master/config-linux.md#control-groups

[sameo]

Most of the resource constraints would need to be applied to the qemu process, a.k.a. the VM, by either using cgroups on the host or map the restrictions to qemu parameters (CPU, memory...).

[sameo]

This issue was moved to clearcontainers/runtime#19