Mount Cgroup fs manually

Mount cgroup subsystem fs if needed, and unmount all cgroup fs when cluster disabled.
Temporary reboot implementation of LXC hypervisor
Introduce instance variable _run_cmd_fn for LXCHypervisor to make debug and test easy
Introduce _GetCgroupInstanceValue in LXCHypervisor to read cgroup parameter value from unified interface and remove codes violating DRY rule
Move codes unmounting mounted dirs from StopInstance to CleanupInstance

Because we need to call StopInstance without doing clenaup tasks from RebootInstance().
Remove useless codes from LXCHypervisor.RebootInstance()

No need to re-mount rootfs. Just simply destroy container process and
respawn it.
Remove useless method LXCHypervisor._GetCgroupSubsystemMountPoint