virtualization script should validate cloud images it downloads

There are two different scripts that need to be adjusted for this.

First, maniacs-setup from maas-cert-server makes a local mirror of cloud images on the MAAS server to speed deployment.

maniacs-setup should also download the md5sums or sha256sums file from cloud-images.u.c at the time it downloads the cloud images.

It should then sum the downloaded images and validate that they match.

Next, the virtualization script in plainbox-provider-checkbox is also capable of pulling down images from the MAAS server or cloud-images.

This one is more tricky. it should do the following:

IF cloud image comes from cloud-images.u.c, it should also grab the md5sums from there and validate the download.

IF cloud image comes from non-u.c. site, it shoudl ATTEMPT to get the md5sums, and if that succeeds, then validate, if not, throw a warning and carry on hoping the image is good.

IF the image is supplied locally, assume it's valid and carry on without validating the md5sum.