feat: add support for N nginx ingresses per service

[tperdue321]

This addresses Rally's need to support multiple stable ingresses per rollout in argo rollouts.

An issue against the primary repo has been made but not a lot of feedback on it.
argoproj#2001

Modifications:

• update the Nginx datatype to have a field called AdditionalStableIngresses to allow for 1-N stable ingresses instead of just 1.
• modify the SetWeight method to create a canary for each stable ingress and apply new weight to all canary ingresses.
• update validations to validate all stable ingresses.
• designed to be fully backwards compatible

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[ssanders1449]

Travis - This is exactly the feature we need for one of our products. Thanks for the effort.
Have you tried to get this merged into the master? I will try to push it with the community