model migration credential check too restrictive

[Ian Booth]

When migration a model, a check is done using reflect.DeepEquals() of the credential on the target controller vs that on the source controller.

This is too restrictive and can cause a valid migration attempt to fail. In 2.7, k8s credentials gain a new attribute and so even though the target controller can access the k8s cluster just fine, a migration from a 2.6 model is aborted.

If the reason for checking the credential on the target controller is to ensure it can access the resources in the model being migrated, this needs to be done is a less naiive way.

[John A Meinel (jameinel)]

Also related to https://bugs.launchpad.net/juju/+bug/1850579