SNO validation

Currently, we have this validation  https://github.com/openshift/installer/blob/master/pkg/asset/agent/installconfig_test.go#L103 which checks if the platform is none then the number of control planes should be 1 and workers should be zero.

We need another validation to check if the number of control planes is 1 and workers are zero, the in the install-config.yaml the platform can only be set as none and in agent-cluster-install.yaml, the platformType should only be set as none. If we try to do SNO (i.e. control planes is 1 and workers are zero)  with e.g. platform: baremetal then assisted will reject it, so we should catch it as early as possible