Added ExtManagementSystem to the TAG_CLASSES list.

Added ExtManagementSystem to the TAG_CLASSES list so Cloud/Infrastructure Provider.My Company Tags: * can be displayed in list of fields in reports editor for the models that have relationships to ext_management_system.

https://bugzilla.redhat.com/show_bug.cgi?id=1160878
https://bugzilla.redhat.com/show_bug.cgi?id=1160879

@dclarizio please review/test.