slapos_cloud: Update constraints to prevent set improper data on request

   Try to guarantee that data is consistent and wont 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