Add credential id in instance group

[quentinleclerc]

Hello,
Added the possibility to specify a credential_id referring to a Kubernetes or OpenShift credential to create ContainerGroup.

Also I removed the instances field from the types in goawx because:

1. the AWX API says it's a 'field' (aka ManyToMany object in Django) but really all I've seen in my AWX instances is an integer (the number of instances in the group) and thus there is a conflict between the type int returned by the API and the type in the goawx (which would need big refactor, don't really know how to manage this).
2. It's not a field that can be updated via the API endpoint "instance_groups" but rather with the "instance_groups/id/instances", and it's not an information used/needed when managing instance_groups via the provider for now.

Thanks