slapos_cloud: Update constraints to prevent set improper data on request
Try to guarantee that data is consistent and won't fail later on while been converted to XML with special attention to non-XML compatibles characters, like backspace.
Include constraint to ensure title is XML compatible on Software Instance, Instance Tree and Slave Instance. (Not required for Compute Node as far as I checked).
Copy text_content (parameter), sla_xml constraints from Software Instance to Instance Tree and Slave Instance.
Copy connection_xml constraint from Software to Slave Instance for consistency.
Add more tests to ensure that an Instance cannot be created if data is invalid, or not XML compatible.
Add utility scripts to validate XML and XML Marshaller compatibility