Enable polling minimization

In some cases the OVS agent might end up wiring a port for a VM
several seconds after the VM has booted. As a result, there is a
risk that the VM won't receive an IP address from DHCP in time.

This patch changes the default value for agent.minimize_polling to
True. This change should reduce the time needed for configuring
an interface on br-int consistently.

Change-Id: I009f606fd34a132376f1d50f8ccda9d35d064bfa
Related-bug: 1224001
Merge "Enable polling minimization"