Disable listen only on ipv6 and fix proxy_protocol

[gianrubio]

• Always listen on ipv4 address for port 443
• Rollback previous PR proxy_protocol on ssl_passthrough listener #227 that broke the proxy_protocol when passthroughBackends is disabled

[coveralls]

Coverage increased (+0.04%) to 45.733% when pulling fda3548 on gianrubio:patch-1 into fb8e2d7 on kubernetes:master.

[aledbf]

@gianrubio this {{ if $cfg.UseProxyProtocol }} proxy_protocol {{ end }} {{ if eq $index 0 }} ipv6only=off{{end}} must be inside the else (443) because if there is passthroughBackends we cannot use proxy_protocol (it was already decoded)

[gianrubio]

@aledbf I did the first change, the second (ipv6only=off) is required for both ports.

[coveralls]

Coverage increased (+0.04%) to 45.733% when pulling 90fdea7 on gianrubio:patch-1 into fb8e2d7 on kubernetes:master.

[k8s-reviewable]

This change is 

[aledbf]

/lgtm