Support for proxy_headers_hash_bucket_size and proxy_headers_hash_max_size

[bashofmann]

This makes proxy_headers_hash_bucket_size and proxy_headers_hash_max_size configurable in the nginx controller.

See https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_headers_hash_bucket_size and https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_headers_hash_max_size

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[k8s-reviewable]

This change is 

[coveralls]

Coverage increased (+0.04%) to 44.357% when pulling 38d198b on bashofmann:master into eb5a530 on kubernetes:master.

[coveralls]

Coverage increased (+0.04%) to 44.357% when pulling 38d198b on bashofmann:master into eb5a530 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@bashofmann thanks!