Irrelevant vsphere platform data is required

This is a clone of issue OCPBUGS-6270. The following is the description of the original issue:
—
Similar to how, due to the install-config validation, the baremetal platform previously required a bunch of fields that are actually ignored (OCPBUGS-3278), we similarly require values for the following fields in the platform.vsphere section:

• vCenter
• username
• password
• datacenter
• defaultDatastore

None of these values are actually used in the agent-based installer at present, and they should not be required.

Users can work around this by specifying dummy values in the platform config (note that the VIP values are required and must be genuine):

platform:
  vsphere:
    apiVIP: 192.168.111.1
    ingressVIP: 192.168.111.2
    vCenter: a
    username: b
    password: c
    datacenter: d
    defaultDatastore: e