Fix the error status code of duplicated agents

If posting the duplicated agent info to "create a agent" API, Nova v2
API returns a HTTP500 response(Internal Server Error).
The duplicated resources are not internal errors and this behavior has
been fixed on Nova v3 API. In this case, Nova v3 API returns a HTTP409
response(Conflict).
This patch fixes it for Nova v2 API.

Change-Id: I0b9424f9792270f6807cabcbf27a0f6e5ec42669
Merge "Fix the error status code of duplicated agents"