Add support for external cluster providers

[OriHoch]

An example provider for Kamatera: https://github.com/OriHoch/cco-provider-kamatera (using rke)

Additional changes:

• Build and push the minikube ckan-cloud-operator image
• Remove --interactive flag is interactive.yaml is used
• Finalize the development docs

[OriHoch]

If you want to test it, you can register for a 30 day trial here: https://console.kamatera.com/create

Then you can get an API key from https://console.kamatera.com/keys

let me know by email if you have any problems.. ori AT uumpa DOT com

[akariv]

Thanks @OriHoch, we will review.

[OriHoch]

Add some more change in this commit -

this integrates the custom provider with the ckan-cloud-operator cluster initialize command

added an option to use an external provider manager from providers_manager.get_provider method - it tries to import a module dynamically according to the provider id.