Use goimports for lint checking

[munnerz]

This switches us to use goimports for linting of files. It performs the same role as gofmt, plus also checks import paths and makes sure they are grouped according to guidelines.

For reference, the 'correct' format:

import (
    // stdlib packages
    "fmt"
    "os/exec"

    // external packages
    "github.com/golang/glog"
    "k8s.io/apimachinery/pkg/apis/meta/v1"

    // packages belonging to our repo
    "github.com/jetstack/navigator/pkg/apis/v1alpha1"
)

Release note:

NONE

[wallrj]

/lgtm

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