An ansible role to install docker in the OpenStack infra production environment

This also installs a log redirector for syslog ```docker-`` tags.  For
most containers, they can be setup in the compose file with a section
such as:

.. code-block:: yaml

   logging:
     driver: syslog
     options:
       tag: docker-<appname>

**Role Variables**

.. zuul:rolevar:: use_upstream_docker
   :default: True

   By default this role adds repositories to install docker from upstream
   docker. Set this to False to use the docker that comes with the distro.

.. zuul:rolevar:: docker_update_channel
   :default: stable

   Which update channel to use for upstream docker. The two choices are
   ``stable``, which is the default and updates quarterly, and ``edge``
   which updates monthly.

.. zuul:rolevar:: force_docker_io_ipv4
   :default: False

   Force all communication to docker.io over IPv4.  We do this as rate-limiting
   is per IPv4 address as opposed to a /64 with IPv6.

.. zuul:rolevar:: docker_registry_hostlist
   :default: ['registry-1.docker.io', 'docker-images-prod.6aa30f8b08e16409b46e0173d6de2f56.r2.cloudflarestorage.com', 'docker.io']

   List of hosts to query for IPv4 addresses and add to /etc/hosts.  Used when
   ``force_docker_io_ipv4`` is ``True``