Make -u option for fab more prominent

I missed the last paragraph when trying to deploy Puppet and was
prompted for a password because my local username differs from my
account on this box.

Make the -u option more prominent to avoid this in future.
I don't think this one is correct because we only have an `ubuntu` account, which [`firstrun` sets the username as](https://github.com/alphagov/govuk_offsitebackups-puppet/blob/master/fabfile.py#L43), the very first time a machine is provisioned.