Add systemd ulimit override function and set an override for NOFILE

This sets our default ulimit NOFILE to 2048, which is double what we
set things like mysql'd max_connections to.

Change-Id: I5126bed1e6b9f8c64db00eae4151ac61e47b1bf8
diff --git a/functions b/functions
index f842f63..bc2d26b 100644
--- a/functions
+++ b/functions
@@ -720,6 +720,22 @@
 }
 
 
+# Set a systemd system override
+#
+# This sets a system-side override in system.conf. A per-service
+# override would be /etc/systemd/system/${service}.service/override.conf
+function set_systemd_override {
+    local key="$1"
+    local value="$2"
+
+    local sysconf="/etc/systemd/system.conf"
+    iniset -sudo "${sysconf}" "Manager" "$key" "$value"
+    echo "Set systemd system override for ${key}=${value}"
+
+    sudo systemctl daemon-reload
+}
+
+
 # Restore xtrace
 $_XTRACE_FUNCTIONS