1. This is a bug because of latest modify about scheduler's instance_group_predicate.
Should set minFree as zero when founding that host can't be used to create vm
with requested forced instance group.
2. Fix some little error of instance group.
3. Add PerformEnable and PerformDisable for instance group.
4. Clean scheduler desc cache after having attached or detached guests and groups
and enabled group. Cleaning was necessary when disable group because disabled
group is prohibited.
5. Add package 'golang.org/x/sync/errgroup'.
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.
enough resources of InstanceGroup and then to host even if this hosts has
no enough resources. Add groups desc in SGuest.ToSchedDesc. The backup
guest also occupies InstanceGroup resources now.