Feature:
1. Add apis: list server by group, list group by server, bind
servers, unbind servers, bind groups, unbind groups.
2. Add climc command and docs corresponding to the above newly added apis
3. Add log about binding and unbinding operator.
4. Support enable and disable instance group.
Fix:
1. Set capacity zeor if there are not enough resource aboue forced
instance group in one host.
2. Change the stage(real delete => pending delete) that guest leave all groups.
Rain