Allow Cluster/InfraEnv registration without configuration

Modify the cluster registration code in the assisted-service client (used by create-cluster-and-infraenv.service) to allow creating the cluster given only the following config manifests:

• ClusterImageSet
• InfraEnv

If the following manifests are present, data from them should be used:

• AgentPullSecret
• NMStateConfig
• extra manifests

Other manifests (ClusterDeployment, AgentClusterInstall) will not be present in an interactive install, and the information therein will be entered via the GUI instead.

A CLI flag or environment variable can be used to select the interactive mode.