[nginx-ingress-controller] Change structure of packages

[aledbf]

Goals of this PR:

• change the layout of the packages to be agnostic to the backend (first step)
• remove most of the references to nginx
• remove unnecessary function exports
• add test
• refactor how the status of the Ingress rules are keep up to date (using leader election)
• move main inside cmd package

My wish is that to implement a new controller (like caddy) you just need to write a template for the tool, calling the generic code and a Dockerfile

replaces #1762

fixes #1665
fixes #1799

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This change is

[mqliang]

cc @ddysher @pendoragon

[aledbf]

@bprashanth ready for a first review

[kayrus]

@aledbf why this PR was closed?

[aledbf]

@kayrus because I'm working in the next version here. When ready I will open a PR in this repo.