[4626] Changes after review

[4626] Changes after review
 - Dhcp4Srv::vendorClassSpecificProcessing removed
 - User's Guide updated
 - disabled obsolete test (will need to be rewritten to take advantage
   of new classification code)
 - classes definitions now use strings for server-name and filename
 - unused configuration removed from unit-tests
 - textFixedFields is now documented
 - Unit-tests now have short descriptions