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/meta/main.yml

@@ -5,3 +5,6 @@ dependencies:
     when: ansible_os_family != 'CoreOS'
   - role: download
     file: "{{ downloads.etcd }}"
+    when: etcd_deployment_type == "host"
+  - role: docker
+    when: (ansible_os_family != "CoreOS" and etcd_deployment_type == "docker")