When cinder-api runs behind a load balancer(eg haproxy), the load
balancer can return 504 Gateway Timeout when cinder-api does not
respond within timeout. This change ensures nova retries deleting
a volume attachment in that case.
Also this change makes nova ignore 404 in the API call. This is
required because cinder might continue deleting the attachment even if
the load balancer returns 504. This also helps us in the situation
where the volume attachment was accidentally removed by users.
Closes-Bug: #1978444
Change-Id: I593011d9f4c43cdae7a3d53b556c6e2a2b939989