Switch git lb to source balance method

We were using the leastconn method which sends new connections to the
backend with the least number of connections. Unfortunately git clients
seem to have trouble with varying backend repo state (due to GC and
packing) and the thought is sending all requests from a single client to
a single backend will alleviate this.

To do this we switch to the source balance method which hashes the
source IP and finds a stable backend to talk to. This method handles
backend outages fine as it will hash to a new backend if the older one
goes offline.

Change-Id: I2c7a4ec0809a2f4ef6556833ac6a0ff3651904dd

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

@@ -25,7 +25,7 @@ listen {{ listener.name }}
   bind {{ bind }}
   {% endfor %}
   mode tcp
-  balance leastconn
+  balance source
   option tcplog
 
   {% for server in listener.servers %}