feature 7919 - Subscription API for OSS/BSS management systems

Delete_extra used to remove mapped_subscription
Moved code to use db.create_list function
Addressed review comments.

Change-Id: I84bd39e9a9c942d15762d4715843c7c539842767
Signed-off-by: preethika.p <preethika.p@tataelxsi.co.in>