Add optional deployment mode for Docker etcd_deployment_type

Running etcd in Docker reduces the number of individual file
downloads and services running on the host.

Note: etcd container v3.0.1 moves bindir to /usr/local/bin

Fixes: #298

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

@@ -0,0 +1,18 @@
+[Unit]
+Description=etcd
+
+
+[Service]
+User=etcd
+EnvironmentFile=/etc/etcd.env
+{% if inventory_hostname in groups['etcd'] %}
+ExecStart={{ bin_dir }}/etcd
+{% else %}
+ExecStart={{ bin_dir }}/etcd -proxy on
+{% endif %}
+Restart=always
+RestartSec=10s
+LimitNOFILE=40000
+
+[Install]
+WantedBy=multi-user.target