Add config for X-Forwarded-For trust

[phekmat]

Use the same config option for set_real_ip_from when not using proxy protocol. The default remains 0.0.0.0/0, which is insecure if the ingress is publicly accessible (or its upstream doesn't strip the X-Forwarded-For header). This at least provides a workaround for #200

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[k8s-reviewable]

This change is 

[phekmat]

CLA should be signed

[coveralls]

Coverage decreased (-0.03%) to 46.953% when pulling dd894f0 on phekmat:patch-1 into 88ca7a5 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@phekmat thanks!