Persist CNI working directory across reboot.

Currently, CNI working directory doesn't persist across reboot due to which CNI isolator loses the network related information and skips doing a proper cleanup. However, this breaks certain CNI network providers that depend on CNI_DEL call to do the necessary cleanup. The fix would be to move the CNI working directory to a location that persists across reboot such as /var/lib/mesos. 

This would be a breaking change from a backward compatibility standpoint and so should be protected by a flag