Add the possibility of automatically managed many-to-many relationships between objec...

Add the possibility of automatically managed many-to-many relationships between objects in the datamodel.
1. Add the concept of a many-to-many mapping in the database schema
2. Loosen the existing datamodel sanity checks to permit this new configuration
3. Enforce referential integrity in the mid-level database update functions (remove_row; set_row_in_table)

Also add a set of unit-tests to check referential integity is maintained across create/destroy/modify

Signed-off-by: David Scott <dave.scott@eu.citrix.com>