Allow the user to choose a platform type

Assisted installations default to setting platform: baremetal. Using the ReST API, it is possible to select vsphere (or ovirt) as the platform type. In every case, the actual platform data is filled in by assisted-service, and cannot be specified by the user.

The ClusterDeployment resource (from Hive) contains a Platform field. We could look for a platform specified in this field and set that platform when creating the cluster in the create-cluster-and-infraenv service. If ZTP were ever to support other deployment methods, this would probably be a good choice for that also.

We should probably warn the user if they attempt to put any data inside the platform settings, as this will be ignored. This shouldn't be an error, though, as it would prevent users from using existing install configs. Perhaps it should be an error if they specify a platform we don't support.

Note: https://issues.redhat.com/browse/AGENT-284?focusedCommentId=21019997&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-21019997 

[Pawan]: We can simply use the PlatformType from ACI and then no assisted service client changes are required. We will throw an error if the user provides an unsupported platformType ( aws, gcp, etc)

Ignoring the unwanted Platform settings from install-config.yaml to be handled in https://issues.redhat.com/browse/AGENT-348