validate upstream downloads when building snaps

[kwmonroe]

I almost had a bad day. I ran build-and-release-kubernetes-snaps with a version param of 1.10.0. The correct param is v1.10.0. Missing the v is bad:

https://ci.kubernetes.juju.solutions/job/build-and-release-kubernetes-snaps/337/console

Notice that I made it all the way to the kubernetes-test snap before a failure. Turns out all the binaries pulled during that job were not binaries at all, but rather xml docs telling me "foo doesn't exist".

We don't validate the downloads are what we expect, and snapcraft happily packaged up our garbage and would have pushed them to the store if not for the lucky failure when preparing kubernetes-test (lucky because the snapcraft prepare phase tried to untar a xml doc).

We should do a simple file validation to ensure what we download from upstream is in fact what we expect.

[kwmonroe]

Closed in #15