Enable custom election id for status sync.

[danielqsj]

What this PR does / why we need it:

• Multiple deployment/repliaset/daemonset of same ingress controller class can not work correctly in a same namespace.

Which issue this PR fixes:
Closes #350

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[k8s-reviewable]

This change is 

[danielqsj]

I signed it!

[coveralls]

Coverage decreased (-0.03%) to 45.733% when pulling 2d526b2 on danielqsj:master into 376519e on kubernetes:master.

[pieterlange]

Nice one! I was just running into this 👍

One small nit: can't we just default the ElectionId to the value of class, as that should be a unique value anyway when deploying multiple sets of ingress controllers?

[danielqsj]

@pieterlange Yeah, I also thought about it. But due to the document of nginx-ingress-controller recommends to use kubernetes.io/ingress.class: "nginx" , so I use a new param. But I still think reusing "class" is also fine.

[aledbf]

/lgtm

[aledbf]

@danielqsj thanks!