Add support for Azure storage as a publishing backend

This adds a new configuration setting: AzurePublishEndpoints, similar
to the existing S3PublishEndpoints and SwiftPublishEndpoints.

For each endpoint, the following has to be defined:
 - accountName
 - accountKey
 - container
 - prefix

Azure tests require the following environment variables to be set:
 - AZURE_STORAGE_ACCOUNT
 - AZURE_STORAGE_ACCESS_KEY

With either of these not set, Azure-specific tests are skipped.

context/context.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"github.com/aptly-dev/aptly/aptly"
+	"github.com/aptly-dev/aptly/azure"
 	"github.com/aptly-dev/aptly/console"
 	"github.com/aptly-dev/aptly/database"
 	"github.com/aptly-dev/aptly/database/goleveldb"
@@ -379,6 +380,18 @@ func (context *AptlyContext) GetPublishedStorage(name string) aptly.PublishedSto
 			if err != nil {
 				Fatal(err)
 			}
+		} else if strings.HasPrefix(name, "azure:") {
+			params, ok := context.config().AzurePublishRoots[name[6:]]
+			if !ok {
+				Fatal(fmt.Errorf("Published Azure storage %v not configured", name[6:]))
+			}
+
+			var err error
+			publishedStorage, err = azure.NewPublishedStorage(
+				params.AccountName, params.AccountKey, params.Container, params.Prefix)
+			if err != nil {
+				Fatal(err)
+			}
 		} else {
 			Fatal(fmt.Errorf("unknown published storage format: %v", name))
 		}