Add validation to custom built OVAs to avoid throwing an error during create flow

Right now, if a user creates an OVA in vsphere on their own, there is no additional validation that occurs in the CLI to verify that it is a valid one. This results in create/upgrade commands failing, and when doing an upgrade, brings the cluster to an unrecoverable state where the user is forced to recreate the cluster.

Start by tracing the CLI create and upgrade flows for user-built vSphere OVAs. Identify where OVA validity is currently assumed, then verify the intended validation behavior and its failure handling; done means invalid custom OVAs are rejected before create or upgrade can leave a cluster unrecoverable.

Written by the indexing model from the issue text.