Re-tune ETCD performance params

Reduce election timeout to 5000ms (was 10000ms)
Raise heartbeat interval to 250ms (was 100ms)
Remove etcd cpu share (was 300)
Make etcd_cpu_limit and etcd_memory_limit optional.

roles/etcd/templates/etcd-docker.service.j2

@@ -14,7 +14,12 @@ ExecStart={{ docker_bin_dir }}/docker run --restart=on-failure:5 \
 -v /etc/ssl/certs:/etc/ssl/certs:ro \
 -v {{ etcd_cert_dir }}:{{ etcd_cert_dir }}:ro \
 -v /var/lib/etcd:/var/lib/etcd:rw \
---memory={{ etcd_memory_limit|regex_replace('Mi', 'M') }} --cpu-shares={{ etcd_cpu_limit|regex_replace('m', '') }} \
+{% if etcd_memory_limit is defined %}
+--memory={{ etcd_memory_limit|regex_replace('Mi', 'M') }} \
+{% endif %}
+{% if etcd_cpu_limit is defined %}
+--cpu-shares={{ etcd_cpu_limit|regex_replace('m', '') }} \
+{% endif %}
 --name={{ etcd_member_name | default("etcd") }} \
 {{ etcd_image_repo }}:{{ etcd_image_tag }} \
 {% if etcd_after_v3 %}