Correct haproxy's stat unix socket path

Correct haproxy's stat unix socket path

We currently set the haproxy stat socket to /var/run/haproxy.sock.
On Centos/RHEL with selinux enabled this will break:

avc: denied { link } for pid=284010 comm="haproxy"
name="haproxy.sock" dev="tmpfs" ino=330803
scontext=system_u:system_r:haproxy_t:s0
tcontext=system_u:object_r:var_run_t:s0 tclass=sock_file

The blessed/correctly-labeled path is /var/lib/haproxy/stats

Note: I am setting only Partial-Bug because I would still like
to make this a parameter so other distros may just override the path.
But that change is more apt for pike and not for ocata.

Change-Id: I62aab6fb188a9103f1586edac1c2aa7949fdb08c
Patial-Bug: #1671119
(cherry picked from commit 5f8607711bb85150bb9631559f0538254ba5c5cc)