Introduce an upstream-hash-by annotation to support consistent hashing by nginx variable or text

[methodmissing]

This PR consolidates the changesets from 2 distinct fork PRs:

• Introduce an upstream-hash-by annotation to support consistent hashing by nginx variable Shopify/ingress#5
• Fix regressions from the initial pass of the upstream-hash-by annotation Shopify/ingress#6

Generates the following:

host:test user$ kubectl describe ing -n something-staging
Name:            something-web
Namespace:        something-staging
Address:        X.X.X.X
Default backend:    default-http-backend:80 (Y.Y.Y.Y:8080)
TLS:
  star-something terminates something-staging.something.com
Rules:
  Host                    Path    Backends
  ----                    ----    --------
  something-staging.something.com    
                             something-web:80 (<none>)
Annotations:
  auth-realm:        blah bblah
  auth-secret:        something-basic-auth
  auth-type:        basic
  upstream-hash-by:    $request_uri <<<<<<<<<<<<<<<<<<<<<<<<<<<

upstream something-staging-something-web-80 {
        # Load balance algorithm; empty for round robin, which is the default
        least_conn;

        hash $request_uri consistent; <<<<<<<<<<<<<<<<<<<<
        server Y.Y.Y.Y:8000 max_fails=0 fail_timeout=0;
}

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[k8s-reviewable]

This change is 

[coveralls]

Coverage increased (+0.01%) to 43.783% when pulling b6569af on methodmissing:upstream-hash-by into ab3afe5 on kubernetes:master.

[coveralls]

Coverage increased (+0.04%) to 33.505% when pulling f3d56e3 on methodmissing:upstream-hash-by into 7ffa0ae on kubernetes:master.

[coveralls]

Coverage decreased (-0.0005%) to 33.467% when pulling f3d56e3 on methodmissing:upstream-hash-by into 7ffa0ae on kubernetes:master.

[coveralls]

Coverage increased (+0.1%) to 33.582% when pulling d607cf6 on methodmissing:upstream-hash-by into 7ffa0ae on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@methodmissing thanks!