LGA-1894 - Add external DNS annotations to ingress

[said-moj]

What does this pull request do?

• Add variable placeholders for cluster name and weight in values.yaml
• Add annotations to ingress.yml referencing the variables above
• Add parameters to all deployment scripts for setting (overriding) the cluster name and weight in values.yaml
• Create a ingress-cluster configmap with following values
  • name The cluster name i.e blue or green
  • weight The weighting of traffic that cluster should receive
• Update the circle config to read contents of ingress-cluster configmap and pass it to the deploy scripts as environment variables.

Any other changes that would benefit highlighting?

Similar to https://github.com/ministryofjustice/cla_backend/pull/736

Command used to create configmap: kubectl create configmap ingress-cluster --from-literal=name=blue --from-literal=weight=100

Checklist

• [x] Provided JIRA ticket number in the title, e.g. "LGA-152: Sample title"