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 properties in the
ingress configmap to enable json logging:

log-format-escape-json: "true"
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"}'

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[k8s-reviewable]

This change is 

[coryfklein]

Checking cncf-cla status.

[coveralls]

Coverage decreased (-0.007%) to 46.531% when pulling fa70e5e on coryfklein:master into 816b7f5 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@coryfklein thanks!