Description of the issue
In the VPC settings of the ROSA wizard, when public privacy is selected, the vpc without public subnet are shown in the VPC dropdown menu and are selectable. If such VPC is selected then "Availability Zone" drop-down menu show only disabled entries. That is, probably, not very good from the UX point of view.

Steps to Reproduce:

1. Launch OCM UI staging.
2. Open ROSA wizard as "Classic".
3. Reach to "Networking" step and choose "public" cluster privacy.
4. Check "Install into existing VPC" and click "Next" button.
5. In VPC Settings step, Select a VPC without public subnets.
6. Click on "Availability zone" and verify that the list contains only disabled entries.

Actual results:

VPC without public subnets can be selected.

Expected results:

Probably it would be better to show the VPC without public subnets as disabled in the VPC drop-down menu.