add etc tunning options

https://coreos.com/etcd/docs/latest/tuning.html

etcd_snapshot_count
and
ionice priority

roles/etcd/defaults/main.yml

@@ -22,6 +22,13 @@ etcd_script_dir: "{{ bin_dir }}/etcd-scripts"
 etcd_heartbeat_interval: "250"
 etcd_election_timeout: "5000"
 
+#etcd_snapshot_count: "10000"
+
+# Parameters for ionice
+# -c takes an integer between 0 and 3 or one of the strings none, realtime, best-effort or idle.
+# -n takes an integer between 0 (highest priority) and 7 (lowest priority)
+#etcd_ionice: "-c2 -n0"
+
 etcd_metrics: "basic"
 
 # Limits