Only limit etcd memory on small hosts (#1860)

Also disable oom killer on etcd

roles/etcd/templates/etcd.j2

@@ -9,6 +9,7 @@
   {% if etcd_memory_limit is defined %}
   --memory={{ etcd_memory_limit|regex_replace('Mi', 'M') }} \
   {% endif %}
+  --oom-kill-disable \
   {% if etcd_cpu_limit is defined %}
   --cpu-shares={{ etcd_cpu_limit|regex_replace('m', '') }} \
   {% endif %}