Add metadata to ComputeNode for user to customize and integrate with other cloud system

Add a metadata column to ComputeNode:
1, User can add some customized data to compute node and play with it. Such as write his/her own scheduler filter to filter host;
2, Add some customized data to integrate with other cloud system. Such as add IPMI info to compute node and integrate with user's hardware mangement system.
3, Populate metrics from other monitor system to compute node for further use and UI display.
4, And so on.