awstagdeprovision: Tag 20 load balancers (max) per request

The previous implementation silently ignored errors returned by
lbToAWSObjects.  After inserting some debugging logging and running
against our CI account, I saw:

  DEBU[2018-09-28T21:54:47-07:00] failed to convert to lbObjects: Error fetching tags for load balancers: ValidationError: 1 validation error detected: Value '[ci-op-d7qwcq8r-1d3f3-con, ci-op-d7qwcq8r-1d3f3-ext, ci-op-d7qwcq8r-1d3f3-int, ci-op-274cpi9w-1d3f3-con, ci-op-274cpi9w-1d3f3-ext, ci-op-274cpi9w-1d3f3-int, ci-op-i01107d8-1d3f3-con, ci-op-i01107d8-1d3f3-ext, ci-op-i01107d8-1d3f3-int, ci-op-5c88gcn3-1d3f3-int, ci-op-5c88gcn3-1d3f3-con, ci-op-5c88gcn3-1d3f3-ext, ci-op-pp79f299-1d3f3-con, ci-op-pp79f299-1d3f3-ext, ci-op-pp79f299-1d3f3-int, ci-op-dnflccjn-1d3f3-con, ci-op-dnflccjn-1d3f3-ext, ci-op-dnflccjn-1d3f3-int, ci-op-k94dmy6q-1d3f3-ext, ci-op-k94dmy6q-1d3f3-con, ci-op-k94dmy6q-1d3f3-int, ci-op-px7qd18y-1d3f3-con, ci-op-px7qd18y-1d3f3-ext, ci-op-px7qd18y-1d3f3-int, ci-op-8bx1yb6n-1d3f3-int, ci-op-8bx1yb6n-1d3f3-ext, ci-op-8bx1yb6n-1d3f3-con, ci-op-ms0zyi8r-1d3f3-ext, ci-op-ms0zyi8r-1d3f3-int, ci-op-ms0zyi8r-1d3f3-con, ci-op-w7w2z53b-1d3f3-ext, ci-op-w7w2z53b-1d3f3-con, ci-op-w7w2z53b-1d3f3-int, ci-op-hd2bkkcr-1d3f3-int, ci-op-hd2bkkcr-1d3f3-ext, ci-op-hd2bkkcr-1d3f3-con, ci-op-9jv0t7ib-1d3f3-int, ci-op-9jv0t7ib-1d3f3-con, ci-op-9jv0t7ib-1d3f3-ext, ci-op-kr1g9hr3-1d3f3-int, ci-op-kr1g9hr3-1d3f3-ext, ci-op-kr1g9hr3-1d3f3-con, ci-op-k5bn7vbb-1d3f3-con, ci-op-k5bn7vbb-1d3f3-int, ci-op-k5bn7vbb-1d3f3-ext, ci-op-p6qisqvk-1d3f3-ext, ci-op-p6qisqvk-1d3f3-int, ci-op-y5zj1kvd-1d3f3-int, ci-op-y5zj1kvd-1d3f3-con, ci-op-p6qisqvk-1d3f3-con, ci-op-y5zj1kvd-1d3f3-ext]' at 'loadBalancerNames' failed to satisfy constraint: Member must have length less than or equal to 20

With this commit, I split the load balancers up into groups of 20
(max) when building the list of tagged objects.  I also log any errors
that crop up.

Along these same lines, DescribeLoadBalancers itself is paginated, but
the default page size (and also the maximum page size) is 400 [1].  So
I'm punting on that for now.

[1]: https://docs.aws.amazon.com/sdk-for-go/api/service/elb/#DescribeLoadBalancersInput

contrib/pkg/awstagdeprovision/awstagdeprovision.go

@@ -307,9 +307,18 @@ func deleteLBs(awsSession *session.Session, filters awsFilter, clusterName strin
 			return false, nil
 		}
 
-		lbObjects, err := lbToAWSObjects(results.LoadBalancerDescriptions, elbClient)
-		if err != nil {
-			return false, nil
+		lbObjects := []awsObjectWithTags{}
+		for i := 0; i < len(results.LoadBalancerDescriptions); i += 20 {
+			j := i + 20
+			if j > len(results.LoadBalancerDescriptions) {
+				j = len(results.LoadBalancerDescriptions)
+			}
+			new, err := lbToAWSObjects(results.LoadBalancerDescriptions[i:j], elbClient)
+			if err != nil {
+				logger.Errorf("error converting load balancers to internal AWS objects: %v", err)
+				return false, nil
+			}
+			lbObjects = append(lbObjects, new...)
 		}
 
 		filteredResults := filterObjects(lbObjects, filters)