Add flags to customize listen ports and detect port collisions

[aledbf]

replaces #931
fixes #56
fixes #620

[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.2%) to 44.327% when pulling 0459674 on aledbf:pc into b731e94 on kubernetes:master.

[srampal]

With this fix, is the functionality complete to allow arbitrary ports (not just 80, 443) to be used in the URL string ? (eg https://a.com goes to service1 and https://a.com:1000 goes to service2). Or is that still not possible even after this fix ?

[aledbf]

@srampal no this change allows custom fixed ports to run multiple ingress controllers with different ports.

[rajasaur]

@aledbf , to achieve what @srampal asked (having https://a.com go to SvcA and https://a.com:10000 go to SvcB), would it be possible to feed in a custom nginx.tmpl (since my requirement is just to service the same URL one at port 80 and one at port 10000, but to go to different services) as mentioned in https://github.com/kubernetes/ingress-nginx/blob/master/docs/user-guide/custom-template.md ?