Use apt-get dist-upgrade during image build.

* devstack-vm-update-image.py: Previously apt-get upgrade was used but
this does not update all desired packages like the kernel. Perform a
dist-upgrade instead to get everythin.

Change-Id: I948e2a89d4c5655e7e52a46b5734825508028cfc
Merge "Use apt-get dist-upgrade during image build."