Update instance network info cache to include vif_type.

vif_type is a new param in legacy_nw_info added in grizzly, and when upgrading
from folsom to grizzly, the existing instance's network info cache in the db doesn't
contain this param, which is needed by vif driver to plug vif to instance. Nova
compute will try to plug the vif when it starts the instance, so we need to update
the existing instance's network info cache before pluging instance's VIF.

fix bug #1152426

Change-Id: I1b839bf791b402b933354d9c17c5713fde21ab09
(cherry picked from commit 45e65d8c0301da689de1afcbc9f45756e71097ab)