Cloud Plugin: handle version checks with new docker repository options

If you try to upgrade your installation image and version at the same time and you don't use `mattermost/mattermost-enterprise-edition` for the image you see this error:

```
Error: cloud-2021-02-25-1 is not a valid docker tag for repository mattermost/mattermost-enterprise-edition
```

This was the command I ran for reference:

```
/cloud upgrade import-me --image=mattermost/mm-ee-cloud --version=cloud-2021-02-25-1
```