haproxy external-check health check build fail

[joshhighet]

it was reported that adding preserve-env in haproxy.j2 resolves this

ref: haproxy/haproxy#2380

multisocks-haproxy-1             | [NOTICE]   (1) : haproxy version is 2.9.0-fddb8c1                                                                                                                                       multisocks-haproxy-1             | [ALERT]    (1) : config : parsing [/usr/local/etc/haproxy/haproxy.cfg:2] 
 'external-check' only supports 'preserve-env' as an argument, found ''.
multisocks-haproxy-1             | [ALERT]    (1) : config : Error(s) found in configuration file : /usr/local/etc/haproxy/haproxy.cfg
multisocks-haproxy-1             | [ALERT]    (1) : config : Fatal errors found in configuration.
multisocks-haproxy-1 exited with code 1