Allow user to configure platform settings on day 1

Currently, the contents of the platform settings in the install-config used to install the cluster (other than the VIPs) are generated by assisted-service. While the user can select the platform type (either baremetal, vsphere, or ovirt) through the assisted API (and the agent installer exposes this), there's no direct way to configure the other platform settings. (ZTP also ignores all platform information in the Hive ClusterDeployment resource.)

The default baremetal settings set the provisioning network to Disabled, so only virtualmedia hosts can be used. The default vsphere settings have placeholders for all of the values.

To allow the user to configure these values on Day 1, we could take the Platform settings from the ClusterDeployment manifest and apply them to the cluster using an InstallConfigOverride.

If ZTP wanted to use the same mechanism in future, it should use the same interface, but possibly may not be able to use InstallConfigOverride, since it also exposes that to users. This may be a case where a new ReST API is needed.