Add scripts to release from travis-ci

[aledbf]

What this PR does / why we need it:

We need this PR to automate the release of images located in this repository using a third party service (travis-ci in this case)

To release an image we just require a personal travis-ci token with access to kubernetes/ingress-nginx and the name of the component to be released:

export TRAVIS_CI_TOKEN=xxxxxxx
export COMPONENT=nginx-slim

curl -sSL https://raw.githubusercontent.com/kubernetes/ingress-nginx/master/.travis/publish-from-travis.sh | bash

Edit: the change of registries can be reverted. This is just to show the release procedure

[k8s-reviewable]

This change is 

[nicksardo]

/assign @bowei @ixdy

[coveralls]

Coverage remained the same at 33.49% when pulling 47ab12a31c171acd30ce1492b02782431383b3aa on aledbf:publish-scripts into d965ee8 on kubernetes:master.

[bowei]

@aledbf -- it would be great if we could move releasing of ingress-nginx off of gcr.io/google_containers to a k8s contributor controlled repo. That way releases would not be a manual process. @thockin

[aledbf]

it would be great if we could move releasing of ingress-nginx off of gcr.io/google_containers to a k8s contributor controlled repo. That way releases would not be a manual process

No problem with that. Just tell me what registry I need to use.

[aledbf]

@bowei besides that, it makes the change to publish the images?

[coveralls]

Coverage decreased (-0.04%) to 33.452% when pulling 42dc4108e29389b13cf6c86aee25310961d3745a on aledbf:publish-scripts into d965ee8 on kubernetes:master.

[coveralls]

Coverage remained the same at 33.389% when pulling 7f85e5cc46295a6c2099ecaa87639f2fd08d80a1 on aledbf:publish-scripts into f8c2fb9 on kubernetes:master.

[coveralls]

Coverage decreased (-3.0e-05%) to 33.389% when pulling db04359 on aledbf:publish-scripts into f8c2fb9 on kubernetes:master.

[coveralls]

Coverage remained the same at 33.352% when pulling febd7cf on aledbf:publish-scripts into cd37f96 on kubernetes:master.

[coveralls]

Coverage increased (+0.04%) to 33.389% when pulling febd7cf on aledbf:publish-scripts into cd37f96 on kubernetes:master.