Add backend source port to haproxy logs

When forwarding TCP sockets at OSI layer 4 with haproxy, it helps to
know the ephemeral port from which it sources each connection to the
backend. In this way, backend connections can be mapped to actual
client IP addresses by correlating backend service access logs with
haproxy logs.

Add "[%bi]:%bp" between the frontend name and backend name values
for the default log-format documented here:

    https://www.haproxy.com/blog/haproxy-log-customization/

Change-Id: Ic2623d483d98cd686a85d40bc4f2e8577fb9087f

playbooks/roles/haproxy/templates/haproxy.cfg.j2

@@ -7,6 +7,7 @@ global
   stats  socket /var/haproxy/run/stats uid 1000 gid 1000 mode 0600 level admin
 
 defaults
+  log-format  "%ci:%cp [%t] %ft [%bi]:%bp %b/%s %Tw/%Tc/%Tt %B %ts %ac/%fc/%bc/%sc/%rc %sq/%bq"
   log  global
   maxconn  8000
   option  redispatch