Add use_include param to split haproxy.cfg

Splitting haproxy.cfg into multiple files under /etc/haproxy/conf.d/
allows to add listening services to haproxy one at a time, instead of
the current "all or nothing" way imposed by concat.
Declare all non-primary servers as backups

Add define_backups option to haproxy::balancermember to set "backup"
option for all servers except the first server in the list. This is
required to enforce active-passive failover.