***************Host Aggregate********************
https://docs.openstack.org/nova/queens/user/aggregates.html
nova里的相关指令：
aggregate-add-host Add the host to the specified aggregate.
aggregate-create Create a new aggregate with the specified details.
aggregate-delete Delete the aggregate.
aggregate-details Show details of the specified aggregate.
aggregate-list Print a list of all aggregates.
aggregate-remove-host Remove the specified host from the specified aggregate.
aggregate-set-metadata Update the metadata associated with the aggregate.
aggregate-update Update the aggregate's name and optionally availability zone.
availability-zone-list List all the availability zones.
要点：
++一个host可以定义在多个Host Aggregate里，但只能属于一个Availability Zone。
++Availability Zone是在指令nova aggregate-create中创建的。
++必须先使用nova aggregate-remove-host 指令删除HA中host，然后才能删除该HA