Don't try to generate license when creating cloud subscription

The GenerateLicense function is being called by the CreateSubscription function whenever someone creates a cloud workspace. This is causing some issues because that function isn't meant to generate cloud licenses. Since we don't actually need to generate the license (the provisioner handles this) we should not call the GenerateLicense function for cloud installations.

Here's an example of the error:
```
instance=1q33encyjtyqxdkhszng6dhacc
customer-web-server-646d8db498-n57mr customer-web-server time="2021-02-23T23:30:11Z" level=error msg="failed to generate license for subscription sub_J0AfuEaLEzAQnW" error="failed to get a license features list for the SKU "
```

https://github.com/mattermost/customer-web-server/blob/3e2068af333e1be87fcc50786996ba63f8b64a97/store/licenses.go#L27

skuShortName will always be blank here, because of this if statement

Which means that https://github.com/mattermost/customer-web-server/blob/3e2068af333e1be87fcc50786996ba63f8b64a97/store/licenses.go#L32 won't return anything.