restapi: Split SELinuxMode enum between businessentities and models.

In the patch that introduced the SELinuxMode, same enum was used for
both backend and restapi. This change splits the 2 enums so each may
implement the functionality it needs in api/backend.

Change-Id: I5c09dfcb89f8cdd4c75d8215693779157bf2cd8e
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>