Fixed ServerExist Model

With the latest stacktach deployment in preprod, the ServerExist
now also consists of event_id field. Modified the model to include
the recent development change.

Change-Id: I2e704f9756880bc971b1a5f8f3e565bb17704662
Added mechanism to prevent false test failures based on the
eventual consistency of Swift.

Change-Id: I1b36911071aca4de075cc806dd70291c0599d052
Merge "Added mechanism to prevent false test failures based on the eventual consistency of Swift."