Stop running apt-get during os-refresh-config.

All software installation is meant to happen during image build, not
execution. Cinder was breaking this because of it's requirement for a
dkms module. We added automatic building of dkms modules for all
kernels on the image when we sorted openvswitch out.

Fixes bug 1197371.

Change-Id: I6555a66ebdab3d4d7be927a802898bf483048590