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/defaults/main.yml

@@ -1,3 +1,10 @@
 ---
 etcd_version: v3.0.1
 etcd_bin_dir: "{{ local_release_dir }}/etcd/etcd-{{ etcd_version }}-linux-amd64/"
+
+# Possible values: host, docker
+etcd_deployment_type: "host"
+
+
+etcd_image_repo: "quay.io/coreos/etcd"
+etcd_image_tag: "{{ etcd_version }}"