Fix the update e2e test to remove a race.

See #2361 

The problem occurs when we scale down, and the pod doesn't get deleted quick enough, and then disappears underneath us while we're trying to introspect it.

Solved in two ways:
  * Make a failed get not fail the test
  * Explicitly look for a match to the expected number of replicas.

Something like:
```pod "44439f5e-6b86-11e4-843e-42010af02fc5" not found```

Def. not something that will match the image name we expect, so the next test is guaranteed to fail.

Put another way, the precedence is ```(current_image=$(...)) || true```