Fix systemd configuration of OpenVPN server

The server was not starting.  As a result, the dnsmasq service failed to
start, and the playbook thus failed to run when using the vpn role.
This patch corrects the configuration per instructions from
https://help.ubuntu.com/community/OpenVPN.

OpenVPN PAM configuration moved up to reduce server bouncing as the
playbook runs.  The dependency on service (re)starts between openvpn and
dnsmasq works but feels brittle.

roles/vpn/handlers/main.yml

   service: name=dnsmasq state=restarted
 
 - name: restart openvpn
-  service: name=openvpn state=restarted
+  service: name=openvpn@server state=restarted

roles/vpn/tasks/openvpn.yml

   template: src=etc_openvpn_server.conf.j2 dest=/etc/openvpn/server.conf
   notify: restart openvpn
 
+- name: Copy OpenVPN PAM configuration file into place
+  copy: src=etc_pam.d_openvpn dest=/etc/pam.d/openvpn
+  notify: restart openvpn
+
+- name: Enable OpenVPN server systemd service unit
+  service: name=openvpn@server enabled=yes
+
 # OpenVPN must restart first so the 10.8.0.0 interface is available
 # to dnsmasq
 - meta: flush_handlers
   copy: src=etc_dnsmasq.conf dest=/etc/dnsmasq.conf
   notify: restart dnsmasq
 
-- name: Copy OpenVPN PAM configuration file into place
-  copy: src=etc_pam.d_openvpn dest=/etc/pam.d/openvpn
-  notify: restart openvpn
-
 - name: Copy the ca.crt and ta.key files that clients will need in order to connect to the OpenVPN server
   command: cp {{ openvpn_path }}/{{ item[1] }} {{ openvpn_path }}/{{ item[0] }}
   tags: