This is a role for installing docker on our control-plane servers.
It is based on install-docker from zuul-jobs.
Basic testinfra tests are added; because docker fiddles the iptables
rules in magic ways, the firewall testing is moved out of the base
tests and modified to partially match our base firewall configuration.
Change-Id: Ia4de5032789ff0f2b07d4f93c0c52cf94aa9c25c