roles/dch-gw: Ensure dhcpcd starts after network

`dhcpcd` needs to start after the `network` service has started, as the
latter creates the interfaces to which the former needs to delegate IPv6
prefixes.

roles/dch-gw/files/dhcpcd-after-network.conf

@@ -0,0 +1,2 @@
+[Unit]
+After=network.service

roles/dch-gw/handlers/main.yml

@@ -1,2 +1,4 @@
+- name: reload systemd
+  command: systemctl daemon-reload
 - name: rebind dhcp leases
   command: dhcpcd -n

roles/dch-gw/tasks/main.yml

@@ -6,6 +6,18 @@
   notify: rebind dhcp leases
 - meta: flush_handlers
 
+- name: ensure dhcpcd unit extension directory exists
+  file:
+    path=/etc/systemd/system/dhcpcd.service.d/
+    mode=0755
+    state=directory
+- name: ensure dhcpcd starts after network
+  copy:
+    src=dhcpcd-after-network.conf
+    dest=/etc/systemd/system/dhcpcd.service.d/after-network.conf
+    mode=0644
+  notify: reload systemd
+
 - name: ensure ipv4 forwarding is enabled
   sysctl:
     name=net.ipv4.conf.all.forwarding