Run command from Jenkins to Linux Server
If you have ssh access from the Jenkins machine to the HAProxy machine, you could run an Ansible task:

ansible -i ha-inventory.ini ha-servers -m service -a name=haproxy state=restarted

Of course you could also have Jenkins do a simple ssh :

for server in ${server_list} ; do ssh server 'sudo service haproxy restart' ; done

For both of these, you would need 


ssh keys set up between Jenkins and the HAProxy servers
sudo rights for restarting the service (at least) for the connecting user (probably jenkins)