Nutanix platform validations run at `create manifests` stage

This is a clone of OCPBUGSM-47085
Version:

$ openshift-install version
4.11.0-rc2

Platform:

Nutanix

On `openshift-installer create manifests` stage a connection to Prism is made (see https://github.com/openshift/installer/blob/master/pkg/asset/installconfig/nutanix/validation.go#L15-L36=)

This make generating manifests separately impossible, which breaks Assisted Installer flow. Instead of storing sensitive user information, Assisted Installer sets fake details in install-config.yaml and asks user to update these after installation has completed.

With validation happening on `openshift-install create manifests` phase installation process can't start with invalid credentials.

Please move this validation to ValidateForProvisioning, similar to vSphere