Store neutron port status in VIF model

This patch adds a field into the VIF model called status
which is True if a port in neutron has a status of 'ACTIVE' or
if a port is marked as admin_state_up=Fase (as the user has marked
the port to be down). Otherwise this value will be false. This is
needed for the admin-event-callback-api work. This allows nova-compute to
check the vif status to determine if it needs to wait on a vif-plugged
event before starting a instance.

Related to blueprint admin-event-callback-api
Change-Id: I25d03739afc1f3d74d2830e08ccb418e8cfd664e
Merge "Store neutron port status in VIF model"