Kolla Ansible's bootstrap-servers command provides support for
installing the Docker engine. This is currently done using the packages
at https://apt.dockerproject.org and https://yum.dockerproject.org.
These packages are outdated, with the most recent packages from May 2017
- docker-engine-17.05.

The source for up to date docker packages is
https://download.docker.com, which was introduced with the move to
Docker Community Edition (CE) and Docker Enterprise Edition (EE).

This change adds support to bootstrap-servers for Docker CE for CentOS
and Ubuntu.

It also adds a new variable, 'enable_docker_repo', which controls
whether a package repository for Docker will be enabled.

It also adds a new variable, 'docker_legacy_packages', which controls
whether the legacy packages at dockerproject.org will be used or the
newer packages at docker.com. The default value for this variable is
'false', meaning to use Docker CE.

Upgrading from docker-engine to docker-ce has been tested on CentOS 7.5
and Ubuntu 16.04, by running 'kolla-ansible bootstrap-servers' with
'docker_legacy_packages' set to 'false'. The upgrades were successful,
but result in all containers being stopped. For this reason, the
bootstrap-servers command checks running containers prior to upgrading
packages, and ensures they are running after the package upgrade is
complete.

As mentioned in the release note, care should be taken when upgrading
Docker with clustered services, which could lose quorum. To avoid this,
use --serial or --limit to apply the change in batches.

Change-Id: I6dfd375c868870f8646ef1a8f02c70812e8f6271
Implements: blueprint docker-ce