Use ServerAssignmentType UNREGISTERED rather than NO_ASSIGNMENT when no IPMI available
Fix UT
Merge pull request #122 from Metaswitch/server_assignment_unregistered

[Reviewer: Mike] Use Server-Assignment-Type UNREGISTERED rather than NO_ASSIGNMENT when no IMPI available
Improve HSS interop
Fix UT
Code review mark-ups
Merge pull request #123 from Metaswitch/hss-interop

[Reviewer: Andy] Improve HSS interop