cli: add config and images command

Extends the existing zot CLI to add commands for listing all images and
their details on a zot server.
Listing all images introduces the need for configurations.

Each configuration has a name and URL at the least. Check 'zot config
-h' for more details.

The user can specify the URL of zot server explicitly while running the
command or configure a URL and pass it directly.

Adding a configuration:
zot config add aci-zot <zot-url>

Run 'zot config --help' for more.

Listing all images:
zot images --url <zot-url>

Pass a config instead of the url:
zot images <config-name>

Filter the list of images by image name:
zot images <config-name> --name <image-name>

Run 'zot images --help' for all details

- Stores configurations in '$HOME/.zot' file

Add CLI README

cmd/zot/main.go

@@ -2,12 +2,20 @@ package main
 
 import (
 	"os"
+	"path"
 
 	"github.com/anuvu/zot/pkg/cli"
 )
 
 func main() {
-	if err := cli.NewRootCmd().Execute(); err != nil {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		panic(err)
+	}
+
+	configPath := path.Join(home + "/.zot")
+
+	if err := cli.NewRootCmd(configPath).Execute(); err != nil {
 		os.Exit(1)
 	}
 }

cmd/zot/main_test.go

@@ -1,6 +1,7 @@
 package main_test
 
 import (
+	"io/ioutil"
 	"testing"
 
 	"github.com/anuvu/zot/pkg/api"
@@ -14,7 +15,8 @@ func TestIntegration(t *testing.T) {
 		c := api.NewController(config)
 		So(c, ShouldNotBeNil)
 
-		cl := cli.NewRootCmd()
+		tempFile, _ := ioutil.TempFile("", "tmp-")
+		cl := cli.NewRootCmd(tempFile.Name())
 		So(cl, ShouldNotBeNil)
 
 		So(cl.Execute(), ShouldBeNil)