Only fetch port_id from SG binding table

Change a query to only retrieve the port_id instead of
every column from the row of security group binding info.

Partial-Bug: #1373851
Change-Id: I0fba9c9623898ee52590207ebbb728503bb59a5b
(cherry picked from commit 6acadab5eb8b7b627e097a638d8486bef59a7f30)
Merge "Only fetch port_id from SG binding table" into stable/juno