Fix full XFF with PROXY

[maxlaverse]

What this PR does / why we need it:
The PR #1489 added a new ConfigMap option (compute-full-forwarded-for) that allows us to compute a full X-Forwarded-For header as described in https://tools.ietf.org/html/rfc7239, section 5.2. With this option enabled, the Nginx Ingress adds the remote IP to the list of IPs (empty or not) of the X-Forwarded-For header (header name configurable). This is fine for requests proxied using the HTTP protocol.

When using the PROXY protocol to forward the request to the Nginx Ingress (with use-proxy-protocol=true in the Ingress Controller ConfigMap), with the current implementation the remote IP is added to the list, which is the L4 proxy IP. As discussed in #1489, this is not the expected behavior as:

• we loose the address of the client that connected to the L4 proxy
• L4 proxies act on the TCP level and are supposed to be transparent, meaning they don't appear in the X-Forwarded-For header

This PR will change the behavior of the Ingress when compute-full-forwarded-for and use-proxy-protocol are set to true, and adds the client IP as sent in the PROXY protocol to the X-Forwarded-For header.

Example with compute-full-forwarded-for enabled
With use-proxy-protocol=false, for a client (ip 1.2.3.4) that makes a request to a L7 load-balancer (ip 10.0.0.1) in front of the Ingress, the X-Forwarded-For received by the back-end is 1.2.3.4, 10.0.0.1 (unchanged)

With use-proxy-protocol=true, for a client (ip 1.2.3.4) that makes a request to a L4 load-balancer (ip 10.0.0.1) in front of the Ingress, the X-Forwarded-For received by the back-end would be 1.2.3.4 with this PR. It would have been 10.0.0.1 without.

Special notes for your reviewer:
I used HAProxy in front of the Ingress to test the behavior with the PROXY protocol. I can post the configuration and yaml file if needed.

[k8s-reviewable]

This change is 

[coveralls]

Coverage increased (+0.04%) to 33.135% when pulling b85055a on maxlaverse:fix_fullforwardedfor_with_useproxy into e4e1e68 on kubernetes:master.

[coveralls]

Coverage increased (+0.04%) to 33.135% when pulling b85055a on maxlaverse:fix_fullforwardedfor_with_useproxy into e4e1e68 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@maxlaverse thanks!