allocation duplicate after host-evacuation

[Holger HUO]

After executing `nova host-evacuate --target_host new_node failed_node`, there is a duplicate resource allocation on both failed_node and the new_node.

Steps to reproduce:

1. evacuate all instances with `nova host-evacuate --target_host new_node failed_node`
2. check project allocations from either `/usages?project_id={PROJECT_ID}` or mysql `select * from allocations where consumer id=CONSUMER_ID`

Result: There are two identical records with only resource_provider_id different(from failed and new hosts).

Expected Result: allocation from failed host getting released.

Maybe some race condition in placement, any ideas on further debug?

[Holger HUO (holgerhuo)]

Other information:
1. (Maybe related) we executed `nova host-evacuate --target_host new_node failed_node` twice (`nova host-evacuate --target_host new_node1 failed_node` and `nova host-evacuate --target_host new_node2 failed_node`). The first attempt had no effect for unknown reasons.
2. Another project has two generations of consumer (generation 1 and 4), which also caused allocation to go over actual resource usage. According to the api-ref/src, shouldn't each record has only one generation?