Feature 7919: Added method to check duplicate subscription

New common method for NS, NSD and VNFD subscriptions check_conflict_on_new()
compares new subscription against existing subscriptions. It will reject
new subscription if and only if there exisits a subscription
which has same auth callbackuri, username(if present) and filters.

Change-Id: I0196ee4dfc1207c635cfa4c2ac5798be3579a095
Signed-off-by: K Sai Kiran <saikiran.k@tataelxsi.co.in>