[CE-449] renaming the root device for ansible agent

xvda is, by convention, used for non root devices.
in this case, however, this yaml is describing a
root device, so it should be sda, not xvda

Change-Id: Iff446960d4cacbeb61cf4cdc62f0ba2d353592e2
Signed-off-by: Alex Meijer <alex@corsha.com>

src/agent/ansible/roles/cloud_aws/provision/tasks/apply.yml

@@ -27,7 +27,7 @@
     instance_tags:
       Name: "{{ inventory_hostname }}"
     volumes:
-      - device_name: "/dev/xvda"
+      - device_name: "{{ cluster.root_block_device_name | default('/dev/sda1') }}"
         device_type: "gp2"
         volume_size: "{{ cluster.volume_size }}"
         delete_on_termination: true

src/agent/ansible/vars/aws.yml

@@ -22,7 +22,7 @@ cluster: {
   ssh_user: "ubuntu",
   validate_certs: True,
   private_net_name: "demonet",
-
+  root_block_device_name: "/dev/sda1", # the name by which AWS mounts the root volume as - typically /dev/sda
   public_key_file: "/home/ubuntu/.ssh/fd.pub",
   private_key_file: "/home/ubuntu/.ssh/fd",
   ssh_key_name: "fabric",