Fix unbound variable
Fix unbound variable

With set -u, referencing $HAPROXY_SYSCTL_PARAMS will cause errors like "/marathon-lb/run: line 70: HAPROXY_SYSCTL_PARAMS: unbound variable". This effectively break existing setups.
vixns/marathon-lb,vixns/marathon-lb,mesosphere/marathon-lb,GrovoLearning/marathon-lb,vixns/marathon-lb,knesenko/marathon-lb,knesenko/marathon-lb,matt-deboer/marathon-lb,matt-deboer/marathon-lb,GrovoLearning/marathon-lb,mesosphere/marathon-lb,GrovoLearning/marathon-lb,JamesAwesome/marathon-lb,JamesAwesome/marathon-lb,JamesAwesome/marathon-lb,mesosphere/marathon-lb,matt-deboer/marathon-lb,knesenko/marathon-lb