Issue https://github.com/WindowsAzure/azure-sdk-tools-xplat/issues/586#issuecomment-19298167

Please see the detailed issue in the xplat CLI issue: https://github.com/WindowsAzure/azure-sdk-tools-xplat/issues/586#issuecomment-19298167

This change on the Node service management side updates the JSON-based schema format for roles + updates the reader to introduce a new custom schema metadata type to allow empty XML elements when the metadata value is set, so blah[key] may not evaluate, but an empty XML element will still be generated.

This supports an important scenario for IaaS Linux VMs when it comes to security by offering parity with the Windows Azure Management Portal - to create Linux VMs with a (username, certificate) pair instead of (username, password).

And in case you're wondering: I've validated this one, but in the case that an empty password is provided and no SSH certificate is elected, the REST endpoint validation does the proper thing and complains about not having a password of the appropriate restrictions, length, etc.