framework: occupy

@fengjingchao This works. need some refactoring. 
@xiaoyunwu 
i put the `host:port` to the slot directly for now. 
we need to add another layer of abstraction for node. 
the plan is: 
1. add node layer into framework. so framework knows its own node info
2. framework creates node under /app/node/{id} with node info
3. framework save nodeid into the task instead of `host:port`.