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/configuration/validation_test.rb

@@ -90,10 +90,8 @@ class ConfigurationValidationTest < ActiveSupport::TestCase
 
   test "builder" do
     assert_error "builder: unknown key: foo", builder: { "foo" => "bar" }
-    assert_error "builder/remote: should be a hash", builder: { "remote" => true }
-    assert_error "builder/remote: unknown key: foo", builder: { "remote" => { "foo" => "bar" } }
-    assert_error "builder/local: unknown key: foo", builder: { "local" => { "foo" => "bar" } }
-    assert_error "builder/remote/arch: should be a string", builder: { "remote" => { "arch" => [] } }
+    assert_error "builder/remote: should be a string", builder: { "remote" => { "foo" => "bar" } }
+    assert_error "builder/arch: should be an array or a string", builder: { "arch" => {} }
     assert_error "builder/args: should be a hash", builder: { "args" => [ "foo" ] }
     assert_error "builder/cache/options: should be a string", builder: { "cache" => { "options" => [] } }
   end