Landing job uses pre-push script

[Martin Packman]

The landing job for juju now uses the pre-push script from the juju
branch before running tests. This means it's running go vet over the
source, and I've fixed a couple of errors on master.

What it doesn't do currently is fail the landing if vet reports
issues, as the script doesn't fail in this case. If we want to make go
vet issues gate the landing of code, we can just land that change to
the pre-push script.

Martin