Store LoadBalancerStatus on urlmap instead of storing it on the forwarding rule

[nikhiljindal]

Ref https://github.com/GoogleCloudPlatform/k8s-multicluster-ingress/issues/145

Main changes:

• Stop storing LoadBalancerStatus on forwarding rules
• Start storing status on urlmaps
• Update List and GetStatus to read status from both forwarding rule and urlmap
• Update tests to test that both forwarding rule and urlmaps work fine with and without status.

We wont need special migration for existing MCI's. Gradually as users update their MCI, status will be migrated from forwarding rule to urlmap.

User visible change: If they run kubemci create again with existing ingress spec, they will expect it to not make any change. With this, it will throw an error saying user needs to run the command with --force to update the forwarding rule and urlmap. Will include it in release notes.

cc @csbell @G-Harmon

[nikhiljindal]

Will rebase this once https://github.com/GoogleCloudPlatform/k8s-multicluster-ingress/pull/150 and #151 merge.

[madhusudancs]

/assign

[nikhiljindal]

Thanks for the review @madhusudancs Updated code as per comments. Have pushed a different commit for latest changes.

The new commit also has some changes to FakeForwardingRuleSyncer to fix TestRemoveFromClusters in loadbalancersyncer_test.go due to rebasing on https://github.com/GoogleCloudPlatform/k8s-multicluster-ingress/pull/146

[nikhiljindal]

Thanks for the review @madhusudancs Updated code as per comments. Also squashed commits

[nikhiljindal]

Will merge once the tests pass