Add a script to run 'go vet'

@filbranden 
If dirname fails you won't see this.  We also need to quote $0.  Here is what I've been doing in my pending PR:

```
KUBE_ROOT=$(dirname "${BASH_SOURCE}")/..

# Set the environment variables required by the build.
source "${KUBE_ROOT}/hack/config-go.sh"
```

In my pending change I've standardized on `KUBE_ROOT` as the "root of the world".  I dropped `REPO` as sometimes we aren't running in a git repo (when installing from a tarball).