Corrects the behaviour of default-ssl-certificate

[rikatz]

This PR corrects the behaviour of default-ssl-certificate

WARNING - There is a breaking change here. Now the directive default-ssl-certificate is mandatory, and also it's existence. If there's no default-ssl-certificate the ingress controller deployment will fail.

When the Ingress Controller starts, it tries to fetch a default SSL Certificate to be used on the default backend service.

Also, when a user configures a ingress controller with the 'tls' directive, but doesn't specify the secret to be used, the default will be used instead.

This PR is related to issue #163 and also is a proposal.

[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.1%) to 43.071% when pulling 725f45b on rikatz:defaultsslcert into 40406b1 on kubernetes:master.

[aledbf]

@rikatz please do not make --default-ssl-certificate a mandatory flag and reuse the logic to use the snake-oil-certificate

[rikatz]

@aledbf Tks for the answer. Will see another way to implement this :)

[rikatz]

@aledbf This is WIP. Will continue this on thursday. I don't know if the last changes I've made are enough for this self generated certificate.

I've noticed that Ingress Controller doesn't follow cert updates when you change the 'TLS' directive and so, don't know if this is a bug inserted by me with the modifications, or if it's something pre-existent.

I'll try with the original Master branch the same tests I did here, to see if this still happens.

[rikatz]

OK, so the behaviour is the same without this PR.
Opened the issue #171 , as I'm not able (a poor golang developer here ;P ) to correct this one.

[coveralls]

Coverage decreased (-0.2%) to 43.027% when pulling 07ff578 on rikatz:defaultsslcert into 40406b1 on kubernetes:master.

[rikatz]

@aledbf Closing this PR. Did some mess here. Going to open another one, only with the right updates.

[coveralls]

Coverage increased (+1.003%) to 44.196% when pulling 9b26b9a on rikatz:defaultsslcert into 40406b1 on kubernetes:master.