Use overlay Docker storage driver for seed-base

We no longer need to use devicemapper with a loopback-mounted LVM since
Bifrost can now support the overlay driver.

Change-Id: I043d3be303e337a1fcbbca6e6edc5abc5dcb9c1b

playbooks/kayobe-seed-base/overrides.yml.j2

@@ -12,9 +12,3 @@ kolla_upper_constraints_file: "/tmp/upper-constraints.txt"
 # NOTE(mgoddard): We're using a cirros image, which doesn't require the
 # resolv.conf work around used for CentOS.
 overcloud_host_image_workaround_resolv_enabled: false
-
-# NOTE(mgoddard): Use a loopback-mounted LVM volume for docker storage since
-# the overlay driver doesn't work with the ansible template module until
-# ansible 2.4.0, and this is required by bifrost.
-seed_lvm_group_data_disks:
-  - /dev/loop0

playbooks/kayobe-seed-base/pre.yml

@@ -66,17 +66,6 @@
         src: bifrost-overrides.yml.j2
         dest: "{{ kayobe_config_src_dir }}/etc/kayobe/kolla/config/bifrost/bifrost.yml"
 
-    # NOTE(mgoddard): Create a loopback device backed by a file for docker
-    # storage.  We do this since the overlay driver doesn't work with the
-    # ansible template module until ansible 2.4.0, and this is required by
-    # bifrost.
-    - name: Ensure a docker storage backing file exists
-      command: truncate -s 20G /tmp/docker-storage
-
-    - name: Ensure the docker storage loopback device is created
-      command: losetup /dev/loop0 /tmp/docker-storage
-      become: true
-
     # NOTE(mgoddard): The kayobe dev config by default expects a bridge -
     # breth1 - to exist on the seed with an IP address of 192.168.33.5.
     - name: Ensure all-in-one network bridge interface exists