Improve TLS secret configuration

[jcmoraisjr]

This PR implements the following behavior on ingress.spec.tls[]

---

foo.bar uses existentSecret and bar.baz uses default TLS secret:

  rules:
  - host: foo.bar
    ...
  - host: bar.baz
    ...
  tls:
  - hosts:
    - foo.bar
    secretName: existentSecret
  - hosts:
    - bar.baz

---

foo.bar uses default TLS secret and bar.baz doesn't configure TLS/https:

  rules:
  - host: foo.bar
    ...
  - host: bar.baz
    ...
  tls:
  - hosts:
    - foo.bar

---

foo.bar uses default TLS secret, bar.baz doesn't configure TLS/https and log warn about secret not found:

  rules:
  - host: foo.bar
    ...
  - host: bar.baz
    ...
  tls:
  - hosts:
    - foo.bar
  - hosts:
    - bar.baz
    secretName: nonexistentSecret

[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.06%) to 43.732% when pulling 567041e on jcmoraisjr:jm-improve-tls-secret into f521145 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@jcmoraisjr thanks!