Fix bug 1479 to check conflicts on deleting vnpkg

This change affects descriptor_topics.py, specifically the method
check_conflict_on_del of class VnfdTopic. When deleting a NF package,
several checks are required. One of them is the search of NS packages
using that NF package. The filter used for that search was not updated
when moving to SOL006 descriptors. That filter is updated by this
change.

In addition, this change updates the messages returned in case of
conflicts. The terms 'instance' and 'package' are added for
clarification.

Change-Id: I312e9955347c7b79286a0fb33970441edfc17ff0
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>