[SVC] Delete left network of SNAT service instance

The service monitor should delete left virtual network used by the SNAT
service instance when it destroys it.
That patch add a generic clean method for all instance managers.

Change-Id: Ib2a44c739951c97c3cb7577f86d719b02ee61952
Closes-bug: #1367097
Merge "[SVC] Delete left network of SNAT service instance"