Make log format json escaping configurable

[coryfklein]

This will allow json logging in nginx ingress controllers which greatly
improves ease of log indexing and searching via elasticsearch.

After this change is made, users could set the following property in the
ingress configmap to enable json logging:

log-format-upstream: '{"proxy_protocol_addr": "$proxy_protocol_addr", "proxy_add_x_forwarded_for": "$proxy_add_x_forwarded_for", "remote_user": "$remote_user", "time_iso8601": "$time_iso8601", "request": "$request", "status": "$status", "body_bytes_sent": "$body_bytes_sent", "http_referer": "$http_referer", "http_user_agent": "$http_user_agent", "request_length": "$request_length", "request_time": "$request_time", "proxy_upstream_name": "$proxy_upstream_name", "upstream_addr": "$upstream_addr", "upstream_response_length": "$upstream_response_length", "upstream_response_time": "$upstream_response_time", "upstream_status": "$upstream_status"}'

[k8s-reviewable]

This change is 

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[coryfklein]

Fixing username

[coveralls]

Coverage decreased (-0.007%) to 46.531% when pulling 8c51537 on coryfklein:master into 816b7f5 on kubernetes:master.