increase the maxAttempts for checking namespace deleted. (istio#2179)

Automatic merge from submit-queue.

increase the maxAttempts for checking namespace deleted.

**What this PR does / why we need it**:

I see multiple times for failure of namespace deletion even with this number of attempts, for mixer e2e. This causes a failure of another test for cluster-wide tests, since multiple tests are sharing the same 'istio-system' namespace.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes istio#2157

**Special notes for your reviewer**:

**Release note**:

```release-note
none
```

tests/e2e/framework/kubernetes.go

@@ -197,7 +197,7 @@ func (k *KubeInfo) Teardown() error {
 	}
 
 	// confirm the namespace is deleted as it will cause future creation to fail
-	maxAttempts := 20
+	maxAttempts := 30
 	namespaceDeleted := false
 	for attempts := 1; attempts <= maxAttempts; attempts++ {
 		namespaceDeleted, _ = util.NamespaceDeleted(k.Namespace)