faster quantum-openvswitch-agent periodic reports

One of two patches that fixes bug #1194438.

Instead of spawning ovs-vsctl for every port on br-int, we just dump the
required columns from the Interfaces table and grab the rows that we need. This
is a big win because the time it takes for ovs-vsctl to connect to the
openvswitch controller is orders of magnitude greater than the time it takes to
parse the rows. In practice, instead of taking roughly 1s per interface, the
agent's periodic task only takes 1s in total.

Change-Id: Idbf32c38e0c4a9c9634c1e4f0e79bd556b720493
(cherry picked from commit 364e97b44158976dca923bcf9ff5dff926da5dc4)