DB: Only ask for MAC instead of entire port

Optimize a query in _get_lla_gateway_ip_for_subnet
to only grab the column used instead of every column
in the port table.

Partial-Bug: #1373851
Change-Id: I5257e1e22645f3df9a77c0967b09a0ad0cf8b251
Merge "DB: Only ask for MAC instead of entire port"