add etc tunning options

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

etcd_snapshot_count
and
ionice priority

roles/etcd/templates/etcd.env.j2

@@ -13,6 +13,9 @@ ETCD_NAME={{ etcd_member_name }}
 ETCD_PROXY=off
 ETCD_INITIAL_CLUSTER={{ etcd_peer_addresses }}
 ETCD_AUTO_COMPACTION_RETENTION={{ etcd_compaction_retention }}
+{% if etcd_snapshot_count is defined %}
+ETCD_SNAPSHOT_COUNT={{ etcd_snapshot_count }}
+{% endif %}
 
 # TLS settings
 ETCD_TRUSTED_CA_FILE={{ etcd_cert_dir }}/ca.pem