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.

system/t02_config/ConfigShowTest_gold

@@ -20,5 +20,6 @@
     "skipContentsPublishing": false,
     "FileSystemPublishEndpoints": {},
     "S3PublishEndpoints": {},
-    "SwiftPublishEndpoints": {}
+    "SwiftPublishEndpoints": {},
+    "AzurePublishEndpoints": {}
 }