Configure default security limits in systemd.

Systemd has own configuration of security limits and options setted
in /etc/security/limits.conf are ignored.

Change-Id: I01f8cdcd54a8697bb93951f223ec66838e368804
diff --git a/linux/system/single.yml b/linux/system/single.yml
index 2c538f5..ef23a39 100644
--- a/linux/system/single.yml
+++ b/linux/system/single.yml
@@ -57,3 +57,8 @@
             - type: hard
               item: nproc
               value: 307200
+      systemd:
+        system:
+          Manager:
+            DefaultLimitNOFILE: 307200
+            DefaultLimitNPROC: 307200