Support running multiple instances of the same provider

Up until v1alpha3, the need of supporting multiple credentials was addressed by running multiple instances of the same provider, each one with its own set of credentials while watching different namespaces.

However, running multiple instances of the same provider proved to be complicated for several reasons:

• Complexity in packaging providers: CustomResourceDefinitions (CRD) are global resources, these may have a reference to a service that can be used to convert between CRD versions (conversion webhooks). Only one of these services should be running at any given time, this requirement led us to previously split the webhooks code to a different deployment and namespace.
• Complexity in deploying providers, due to the requirement to ensure consistency of the management cluster, e.g. controllers watching the same namespaces.
• The introduction of the concept of management groups in clusterctl, with impacts on the user experience/documentation.
• Complexity in managing co-existence of different versions of the same provider while there could be only one version of CRDs and webhooks. Please note that this constraint generates a risk, because some version of the provider de-facto were forced to run with CRDs and webhooks deployed from a different version.

Nevertheless, we want to make it possible for users to choose to deploy multiple instances of the same providers, in case the above limitations/extra complexity are acceptable for them.

In order to make it possible for users to deploy multiple instances of the Cluster API controller following flags are provided:

• Providers MUST support the --namespace flag in their controllers.
• Providers MUST support the --watch-filter flag in their controllers.