Include call to delete_subnet from delete_network at DB level

Removes an extra lock in bsn plugin that causes a deadlock
when delete_subnet is invoked form delete_network, agreed
with kevinbenton to remove it
Modifies a unit test to cover this change per reviewer request
Co-author amirosh

Closes-bug: #1197176

Change-Id: Ie3414848a91cc737b16b79399ae19800545e533f
Merge "Include call to delete_subnet from delete_network at DB level"