Fix unittests

Since May 2018, we started offering Start1-XS instances which have only
25GB of SSD. We had to recreate smaller images since most of them were
50GB. `scw create xenial` is now ambiguous because "xenial" and
"ubuntu-mini-xenial-25g" both exist.

The good fix would be to accept creating a server if an exact name
matches. For now I prefer to keep it simple and create
"ubuntu-mini-xenial-25g" instances in unittests (which is unambiguous).

https://blog.online.net/2018/05/03/introducing-scaleway-nextgen-nvme-cloud-servers-with-hot-snapshots/

Author: Julien Castets

pkg/commands/create_test.go

@@ -62,7 +62,7 @@ func TestRunCreate_realAPI(t *testing.T) {
 		Convey("--name=unittest-create-standard wily", func() {
 			args := CreateArgs{
 				Name:           "unittest-create-standard",
-				Image:          "xenial",
+				Image:          "ubuntu-mini-xenial-25g",
 				CommercialType: "X64-2GB",
 				IP:             "dynamic",
 			}