Leverage systemDefaultRegistry setting when creating clusterctl config

[alexander-demicev]

We need to leverage systemDefaultRegistry setting when creating clusterctl config, if this setting is specified Turtles has to use it's value for substituting image registries, if the setting is empty use current defaults.

Proposed solution: we will introduce a feature flag for Turtles that controls whether or not the systemDefaultRegistry setting should be used when pulling down images for the supported providers. The feature flag will be enabled by default, meaning Turtles will, by default, use Rancher's setting as the registry for the images. The clusterctl-config configmap will be used as the source of truth for which providers are supported.