Do not override the complete systemd config

using the drop-in config snipper system, we can add
configuration to systemd without replacing it, in case
the systemd file is changed upstream.

roles/fedmsg/gateway/files/fedmsg-gateway.service

@@ -1,15 +0,0 @@
-[Unit]
-Description=Outbound fedmsg gateway
-After=network.target
-Documentation=http://fedmsg.readthedocs.org/en/latest/commands/
-
-[Service]
-ExecStart=/usr/bin/fedmsg-gateway
-Type=simple
-User=fedmsg
-Group=fedmsg
-LimitNOFILE=160000
-Restart=on-failure
-
-[Install]
-WantedBy=multi-user.target

roles/fedmsg/gateway/files/fs-limits.conf

@@ -0,0 +1,2 @@
+[Service]
+LimitNOFILE=160000

roles/fedmsg/gateway/tasks/main.yml

@@ -23,10 +23,15 @@
   notify:
   - restart fedmsg-gateway
 
-- name: bump fs limits by installing our own systemd service file
+- name: create systemd drop-in directory
+  file: >
+    dest=/etc/systemd/system/fedmsg-gateway.service.d
+    state=directory
+
+- name: bump fs limits by installing a drop-in systemd config
   copy: >
-    src=fedmsg-gateway.service
-    dest=/usr/lib/systemd/system/fedmsg-gateway.service
+    src=fs-limits.conf
+    dest=/etc/systemd/system/fedmsg-gateway.service.d/fs-limits.conf
   tags:
   - config
   - fedmsg/gateway