Implement OpenStack/Swift remote

Rework devcamcar's OpenStack Swift remote [pull
request](https://github.com/hashicorp/terraform/pull/942) to work with
Terraform's new `state/remote` and Gophercloud's current implementation.

`Get()` changed up a bit from devcamcar's version (using different
Gopercloud functionality resulted in less fussing around to figure out
the error case).

Otherwise this is a transliteration/remix of his ideas.

state/remote/remote.go

@@ -40,6 +40,7 @@ var BuiltinClients = map[string]Factory{
 	"consul": consulFactory,
 	"http":   httpFactory,
 	"s3":     s3Factory,
+	"swift":  swiftFactory,
 
 	// This is used for development purposes only.
 	"_local": fileFactory,

state/remote/swift.go

@@ -0,0 +1,114 @@
+package remote
+
+import (
+	"bytes"
+	"crypto/md5"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/rackspace/gophercloud"
+	"github.com/rackspace/gophercloud/openstack"
+	"github.com/rackspace/gophercloud/openstack/objectstorage/v1/containers"
+	"github.com/rackspace/gophercloud/openstack/objectstorage/v1/objects"
+)
+
+const TFSTATE_NAME = "tfstate.tf"
+
+// SwiftClient implements the Client interface for an Openstack Swift server.
+type SwiftClient struct {
+	client *gophercloud.ServiceClient
+	path   string
+}
+
+func swiftFactory(conf map[string]string) (Client, error) {
+	client := &SwiftClient{}
+
+	if err := client.validateConfig(conf); err != nil {
+		return nil, err
+	}
+
+	return client, nil
+}
+
+func (c *SwiftClient) validateConfig(conf map[string]string) (err error) {
+	if val := os.Getenv("OS_AUTH_URL"); val == "" {
+		return fmt.Errorf("missing OS_AUTH_URL environment variable")
+	}
+	if val := os.Getenv("OS_USERNAME"); val == "" {
+		return fmt.Errorf("missing OS_USERNAME environment variable")
+	}
+	if val := os.Getenv("OS_TENANT_NAME"); val == "" {
+		return fmt.Errorf("missing OS_TENANT_NAME environment variable")
+	}
+	if val := os.Getenv("OS_PASSWORD"); val == "" {
+		return fmt.Errorf("missing OS_PASSWORD environment variable")
+	}
+	path, ok := conf["path"]
+	if !ok || path == "" {
+		return fmt.Errorf("missing 'path' configuration")
+	}
+
+	provider, err := openstack.AuthenticatedClient(gophercloud.AuthOptions{
+		IdentityEndpoint: os.Getenv("OS_AUTH_URL"),
+		Username:         os.Getenv("OS_USERNAME"),
+		TenantName:       os.Getenv("OS_TENANT_NAME"),
+		Password:         os.Getenv("OS_PASSWORD"),
+	})
+
+	if err != nil {
+		return err
+	}
+
+	c.path = path
+	c.client, err = openstack.NewObjectStorageV1(provider, gophercloud.EndpointOpts{
+		Region: os.Getenv("OS_REGION_NAME"),
+	})
+
+	return err
+}
+
+func (c *SwiftClient) Get() (*Payload, error) {
+	result := objects.Download(c.client, c.path, TFSTATE_NAME, nil)
+	bytes, err := result.ExtractContent()
+
+	if err != nil {
+		if strings.Contains(err.Error(), "but got 404 instead") {
+			return nil, nil
+		}
+		return nil, err
+	}
+
+	hash := md5.Sum(bytes)
+	payload := &Payload{
+		Data: bytes,
+		MD5: hash[:md5.Size],
+	}
+
+	return payload, nil
+}
+
+func (c *SwiftClient) Put(data []byte) error {
+	if err := c.ensureContainerExists(); err != nil {
+		return err
+	}
+
+	reader := bytes.NewReader(data)
+	result := objects.Create(c.client, c.path, TFSTATE_NAME, reader, nil)
+
+	return result.Err
+}
+
+func (c *SwiftClient) Delete() error {
+	result := objects.Delete(c.client, c.path, TFSTATE_NAME, nil)
+	return result.Err
+}
+
+func (c *SwiftClient) ensureContainerExists() error {
+	result := containers.Create(c.client, c.path, nil)
+	if result.Err != nil {
+		return result.Err
+	}
+
+	return nil
+}

state/remote/swift_test.go

@@ -0,0 +1,25 @@
+package remote
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestSwiftClient_impl(t *testing.T) {
+	var _ Client = new(SwiftClient)
+}
+
+func TestSwiftClient(t *testing.T) {
+	if _, err := http.Get("http://google.com"); err != nil {
+		t.Skipf("skipping, internet seems to not be available: %s", err)
+	}
+
+	client, err := swiftFactory(map[string]string{
+		"path": "swift_test",
+	})
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	testClient(t, client)
+}