Allow a deployer to specify HAProxy syslog server address

Allow a deployer to specify HAProxy syslog server address

This commit aims to allow a deployer to specify where to send haproxy's
logs. It is backward compatible with what is already in place and send
the logs to the UNIX socket /dev/log

The value specified here will be written in the haproxy.cfg file with
the following behavior

HAProxySyslogAddress: 127.0.0.1 -> log 127.0.0.1 local0
HAProxySyslogAddress: ::1 -> log ::1 local0
HAProxySyslogAddress: /dev/log -> log /dev/log local0 (default)

Change-Id: I46c489a1f424e2219d129f332e64c64019aef850
Depends-On: If7f7c8154e544e5d8a49f79f642e1ad01644a66d