Run a mrsk deploy integration test

Adds a simple integration test to ensure that `mrsk deploy` works.

Everything required is spun up with docker compose:
- shared: a container that contains an ssh key and a self signed cert to
be shared between the images
- deployer: the image we will deploy from
- registry: a docker registry
- two vm images to deploy into
- load_balancer: an nginx load balancer to use between our images

The other images are in privileged mode so that we can run
docker-in-docker. We need to run docker inside the images - mapping in
the docker socket doesn't work because both VMs would share the host
daemon.

The docker registry requires a self signed cert as you cannot use basic
auth over HTTP except on localhost. It runs on port 4443 rather than 443
because docker refused to accept that "registry" is a docker host and
tries to push images to docker.io/registry. "registry:4443" works fine.

The shared container contains the ssh keys for the deployer and vms, and
the self signed cert for the registry. When the shared container boots,
it copies them into a shared volume.

The other deployer and vm images are built with soft links from the
shared volume to the require locations. Their boot scripts wait for the
files to be copied in before continuing.

The root mrsk folder is mapped into the deployer container. On boot it
builds the gem and installs it.

Right now there's just a single test. We confirm that the load balancer
is returning a 502, run `mrsk deploy` and then confirm it returns 200.

test/integration/docker/deployer/app/config/deploy.yml

@@ -0,0 +1,14 @@
+service: app
+image: app
+servers:
+  - vm1
+  - vm2
+registry:
+  server: registry:4443
+  username: root
+  password: root
+builder:
+  multiarch: false
+healthcheck:
+  path: /
+  port: 80