Devstack plugin always tries to install docker and etcd

[Salvatore Orlando]

The devstack plugin for kuryr always tries to install docker and etc.
This works fine for the gate but it could a bit annoying for developers who restack several times or use their dev machine for other purposes which might require docker or etcd.

It is true that a developer doesn't actually need devstack to run Kuryr, but since there is a plugin it might make sense for them to use it.

The fix should be fairly simple: treat docker and etcd as services, just like all the other devstack services do with things such as mysql and rabbit, and ovn too (http://git.openstack.org/cgit/openstack/networking-ovn/tree/devstack/plugin.sh#n87)

I am not going ahead to push a fix as that will likely need changes to the jenkins job spec to include the additional services.
If you're ok with doing this I can quickly put together a patch.