ingress-controller: update ingresses only if the load balancer is active

[madumalt]

Issue Description

As per the existing behavior, once the stack update completes, the
ingress controller immediately updates the status of all ingresses and
routegroups to reference the new load balancer. This update may
sometimes happens before the new load balancer has marked its targets
(skipper-ingress) as healthy, leading clients being routed to a load
balancer that is not ready to serve traffic yet.

Fix

To improve this behavior we retrieve the ELBs via aws api and build
the models including the ELB state of the corresponding ELB. Then
before updating the ingresses/routegroups (updateIngress func) we
check whether the ELB is in active state, if not we skip updating
the ingresses/routegroups status with the ELB hostname.

[AlexanderYastrebov]

Overall approach to check loadbalancer status looks OK.
There should be a way to get loadbalancer ARN from CF stack by logical id (which is "LB", see how CF stack is created).
DescribeLoadBalancers API allows fetching multiple load balancers so I think we should collect them first and then buildManagedModel and processes updates.
Please also add tests.

[madumalt]

so I think we should collect them first and then buildManagedModel and processes updates.

made the changes to address this

[madumalt]

TBD

• TestResourceConversionOneToOne tests in worker_test.go is failing
• Add tests for the newly introduced funtions (getLoadBalancerStackResource, GetLoadBalancer)

[szuecs]

this likely needs also an adjustment of our documentation about what AWS policy you need, and if so we would need also to change minor version in VERSION file and communicate the change in the release.

see also https://github.com/zalando-incubator/kube-ingress-aws-controller/blob/master/deploy/requirements.md

[madumalt]

The changes introduced here need permission for elasticloadbalancing:DescribeLoadBalancers and cloudformation:ListStackReources Actions. The requirements.md already have them documents as follows.

{
"Version": "2012-10-17",
"Statement": [
  ...
  {
        "Action": "elasticloadbalancing:DescribeLoadBalancers",
        "Resource": "*",
        "Effect": "Allow"
  },
  ...
  {
        "Action": "cloudformation:List*",
        "Resource": "*",
        "Effect": "Allow"
  },
  ...
]
}

[madumalt]

Closing this PR in favour of #748. I messed the commit history while rebasing, it was easier to cherrypick the changes and create a new PR rather than trying to fix the commit history I messed up here.