Simplify the builders configuration

1. Add driver as an option, defaulting to `docker-container`. For a
   "native" build you can set it to `docker`
2. Set arch as a array of architectures to build for, defaulting to
   `[ "amd64", "arm64" ]` unless you are using the docker driver in
   which case we default to not setting a platform
3. Remote is now just a connection string for the remote builder
4. If remote is set, we only use it for non-local arches, if we are
   only building for the local arch, we'll ignore it.

Examples:

On arm64, build for arm64 locally, amd64 remotely or
On amd64, build for amd64 locally, arm64 remotely:

```yaml
builder:
  remote: ssh://docker@docker-builder
```

On arm64, build amd64 on remote,
On amd64 build locally:

```yaml
builder:
  arch:
    - amd64
  remote:
    host: ssh://docker@docker-builder
```

Build amd64 on local:

```yaml
builder:
  arch:
    - amd64
```

Use docker driver, building for local arch:

```yaml
builder:
  driver: docker
```

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

@@ -23,7 +23,6 @@ registry:
   username: root
   password: root
 builder:
-  multiarch: false
   driver: docker
   args:
     COMMIT_SHA: <%= `git rev-parse HEAD` %>

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

@@ -17,7 +17,6 @@ registry:
   username: root
   password: root
 builder:
-  multiarch: false
   driver: docker
   args:
     COMMIT_SHA: <%= `git rev-parse HEAD` %>